MINERALOGY
Study of naturally occurring crystalline substances (minerals)

Definition of Terms	

•	Crystal – a homogenous solid possessing long-range, three dimensional, internal orders.
•	Mineral – a naturally occurring homogenous solid, with definite chemical composition and an ordered atomic arrangement. It is usually formed by inorganic processes.
•	Rock – is an aggregate of minerals. It can be composed of only one kind of mineral (monomineralic) or of different kinds of minerals.
•	Ore Minerals – those minerals from which one or more metals may be extracted at a profit.
•	Industrial Minerals – those minerals which are, themselves, used for one or more industrial 	purposes such as in the manufacture of electrical and thermal insulators, refractories, ceramics, glass, abrasives, fertilizers, fluxes, cement, and other building materials.
•	Gems – those minerals which have ornamental value, and which possess the qualities of beauty, durability, rarity, fashionability and portability.

HISTORY OF MINERALOGY

Stone Age – the practice of mineralogical arts was started by early humans.  They  made use of flint tools, as shown evidently by cave paintings.  Red hematite and black manganese oxides were used as pigments.

Bronze Age – other minerals were sought from which metals could be extracted.

327-287 BC – Theophrastus, a Greek philosopher had made the first written work on minerals and to Pliny, who recorded 400 years later the mineralogical thought of his time

1556 – Publication of De Re Metallica by the German physician, Georgius Agricola was made. Gives account on mining practices and first factual account of mineral. The book was translated into English from the Latin in 1912 by former Pres. of the United States, Herbert Hoover, and his wife, Lou Henry Hoover.

1669 – Contribution was made to crystallography by Nicholas Steno through his study of the quartz crystal. He noted that despite their differences in origin, size, or habit, the angles between corresponding faces are constant.

1780 – Carangeot invented a device called contact ganiometer for the measurement of interfacial crystal angles

1783 – Rome de I’Lsle made an angular measurement on crystals confirming Steno’s work. Law of the Consistency of Interfacial Angle was formulated

1784 – Rene J. Hauy showed that crystals were built by tiny identical building blocks, which he called integral molecules.

1801 – Hauy developed the Theory of Rational Indences for Crystal Faces

19th Century – rapid advances were made

1809 – Wollaston invented the reflecting ganiometer which permitted highly accurate or precise measurements of the positions of crystal faces

Between 1779-1848 – Berzelius, a Swedish chemist and his students studied the chemistry of minerals and developed the principles of our present chemical classification of minerals.

1815 – Cordier, a French naturalist initiated the immersion method which developed into an important technique for the study of optical properties of mineral fragments. A mineral Cordierite was named after him.

1828 – William Nicol, a Scotsman, invented a polarizing device that permitted the systematic study of the behavior of light in crystalline substances. The device is known today as the polarizing microscope.

Latter part of 19th Century – Federov, Schoenflies, and Barlov developed theories for the internal symmetry and order within crystals which became the foundation of X-ray crystallography.  

1912 – Max Von Laue of the University of Munich suggested Friedrich and Knipping to perform an experiment and demonstrate that crystals could diffract X-rays. X-ray diffraction became a powerful method in the study of minerals.

1913 – W.H. Bragg and W.L. Bragg published the earliest crystal structure determinations

1960 – Electron microprobe helped in the study of chemistry of minerals and is now routinely used for the study of minerals, synthetic compounds, and glasses.

INTRODUCTION TO CRYSTALLOGRAPHY AND MINERAL CRYSTAL SYSTEMS
Steno’s Law – Constancy of Interfacial Angles

Nicholas Steno, a Danish physician and natural scientist, found that, by examination of numerous specimens of the same mineral, when measured at the same temperature, the angles between similar crystal faces remain constant regardless of the size or shape of the crystal.

The Six Crystal Systems

1.	Isometric – (cubic) the three crystallographic axes are all equal in length and intersect at right angles (90o) to each other

c

b

a

a=b=c;α=β=γ=90˚

C

a

b

a=b≠c;α=β=γ=90˚

2. Tetragonal – Three axes, all at right angles, two of which are equal in length (a and b) and one (c) which is different in length.

C

a

b

a≠b≠c;α=β=γ=90˚

3. Orthorhombic – Three axes, all at right angles, and all three of different lengths.

4. Hexagonal – Four axes. Three of the axes fall in the same plane and intersect at the axial cross at 120˚ between the positive ends. These three axes, labeled a1, a2, and a3 are of the same length. The fourth axis, termed c, may be longer or shorter than the axes set. The c axis also passes through the intersection of the axes set at right angle to the plane formed by the a set.

a1

a2

a3

c

a=b≠c;α=β=90˚;γ=120

  Rhombohedral   a=b=c  ; α=β=γ=90˚

5. Monoclinic – Three axes, all unequal in length, two of which (a and c) intersect at an oblique angle (not 90°), the third axis (b) is perpendicular to the other two axes.

c

a

b

a≠b≠c; γ≠α=β=90˚

6.  Triclinic – The three axes are all equal in length and intersect at three different angles ( any angle but 90°).

a≠b≠c; α≠β≠γ90˚

c

a

b

WHAT IS A MINERAL?

	A Mineral is a naturally occurring, homogenous solid with a definite, but generally not fixed chemical composition and an ordered atomic arrangement.  It is usually formed by inorganic processes. 

•	Naturally occurring
	- formed by a natural process
	- Synthetic products or those produced in the laboratory are not considered Minerals
	  Example: synthetic ruby is not a mineral

•	Homogenous solid 
	- consists of a single solid substance that cannot be physically subdivided into simpler chemical compounds
	- excludes gases and liquids

 ex.  Ice in glacier is a mineral but water is not;  liquid mercury fall under mineraloids.
•	Definite chemical composition
	- means that atoms, or groups of atoms must occur in specific ratios. For ionic crystals (i.e. most minerals) ratios of cations to anions will be constrained by charged balance, however, atoms of similar charge and ionic radius may substitute freely for one another, hence definite, but not fixed.

Not fixed. Ex. Dolomite Ca Mg (CO3)2 is not always pure.  It may contain Fe and Mn in place of Mg, therefore chemical formula becomes Ca (Mg,Fe,Mn)(CO3)2.	

•	Ordered atomic arrangement
	- means crystalline, with three-dimensional periodic arrays of precise 		geometric arrangement of atoms.
 		
Example:  both quartz and glass are composed of element Si silicon and O oxygen.  The former has a specific arrangement while the latter have random arrangement.  Glass lacks consistent atomic order and therefore considered as noncrystalline or amorphous.  Another example is graphite 	and diamond.(Polymorphs-same chem. comp. but have different structures)

•	Formed by inorganic process
	- excludes the organically produced compounds
	- ex.  Calcium carbonate in shells, pearl, petroleum and coal
	-  inorganic means pertaining or relating to compound that contains no  carbon.

CRYSTAL HABITS AND AGGREGATES

1.	Minerals in isolated or distinct crystals may be described as:
a)	ACICULAR – slender, needlelike crystals
b)	CAPILLARY and FILIFORM – hairlike or threadlike crystals
c)	BLADED – Elongated crystals flattened like a knife blade

2.	For groups of distinct crystals, the following terms are used:
a)	DENDRITIC – Arborescent, in slender divergent branches, somewhat plant-like
b)	RETICULATED – Lattice-like groups of slender crystals
c)	DIVERGENT or RADIATED – radiating crystal groups
d)	DRUSY – A surface covered with a layer of small crystals  

3.	Parallel or radiating groups of individual crystals, are described as:
a.)	COLUMNAR – Stout, columnlike individuals.
b.)	BLADED  -  an aggregate of many flattened blades
c.)	FIBROUS - aggregate of slender fibers, parallel or radiating.
d.)	STELLATED – radiating individuals forming starlike or circular groups.
e.)	GLOBULAR - radiating individuals forming spherical or hemispherical groups.
f.)	BOTROYOIDAL – globular forms resembling, as the word derived from the Greek word implies, “bunch of grapes”.
g.)	RENIFORM -  radiating individuals terminating in round kidney-shaped masses.
h.)	MAMMILARY - large rounded masses resembling mammae, formed by radiating individuals.
i.)	COLLOFORM – spherical forms composed of radiating individuals without regard to size, this includes botroyoidal, reniform and mammillary.

4.	A mineral aggregate composed of scales or lamellae is described as:
a.) FOLIATED – easily separable into plates or leaves
b.) MICACEOUS – similar to foliated, but splits onto exceedingly thin sheets, as in micas.
c.) LAMMELAR or TABULAR – flat, platelike individuals superimposed upon and adhering to each other.
		d.)  PLUMOSE – fine scales with divergent or featherlike structure.

      5.    A mineral aggregate composed of grains is called GRANULAR
     
      6. Miscellaneous Terms
	a.  STALACTITIC – pendant cylinders or cones. 
b.  CONCENTRIC – more or less spherical layers superimposed upon onme another about a common center.
	c.  PISOLITIC – rounded masses about size of peas.
	d.  OOLITIC - small spheres resembling fish roe.
e.  BANDED – mineral in narrow bandsof different color or texture.
	f.  MASSIVE – compact material without form or distinguishing features.
	g.  AMYGDALOIDAL – a term used to describe a vesicle in a volcanic rock 	that is filled with secondary minerals
	h.  GEODE – rock cavity lined by a mineral mater but not wholly filled.
	i.  CONCRETIONS – masses formed by deposition of materials about a 	nucleus.

DETERMINATIVE MINERALOGY/ PHYSICAL PROPERTIES OF MINERALS
A.	Cleavage – ability of a mineral to break along a definite plane surface.
B.	Parting - ability of a mineral to break along planes of structural weakness
C.	Fracture – the way a mineral breaks when it does not yield along cleavage or parting
1.	Concoidal – smooth curved fracture resembling the surface of a shell
2.	Hackly – jagged fractures with sharp edges
3.	Fibrous or Splintery
4.	Uneven or Irregular – with rough surface
D.	Hardness – the resistance that the smooth surface of a mineral offers to scratching

Moh’s Scale of Hardness
1.	Talc
2.	Gypsum
3.	Calcite
4.	Flourite
5.	Apatite
6.	Orthoclase
7.	Quartz 
8.	Topaz
9.	Corundum
10.	Diamond

E.	Tenacity – the resistance that a mineral offers to breaking, crushing, bending or tearing or its cohesiveness
1.	Brittle – breaks or powders easily
2.	Malleable – can be hammered onto thin sheets
3.	Sectile – can be cut into thin shavings with a knife
4.	Ductile – can be drawn into wire
5.	Flexible – bends but does not resume its original shape when pressure is applied
6.	Elastic – bends and resumes its original shape upon release of pressure

F.	Luster and Transparency – refers to the general appearance of a mineral surface in reflected light. Transparency may either be opaque, translucent, or transparent.
1.	Metallic luster – the brilliant appearance of a mineral like a metal; opaque to light
2.	Non-metallic luster – light-colored and transmits light at least through thin edges
A.	Vitreous – like glass (e.g. Quartz)
B.	Resinous – like the luster of a resin (e.g. Sphalerite)
C.	Pearly – an iridescent pearl-like luster (e.g. Talc)
D.	Greasy – appears as if covered with a thin layer of oil
E.	Silky – silk-like (e.g. Serpentine)
F.	Adamantine – a hard, brilliant luster like that of diamond (e.g. Pb-minerals like Cerussite)

G.	Color – results when white light strikes the surface of a mineral, one part of it is reflected and the other refracted
H.	Streak – the color of a finely powdered mineral; can be determined by rubbing the mineral on unglazed porcelain or a streak plate
I.	Specific Gravity – the relative density (G) is a number that expresses the ratio between the weight of a substance and the weight of an equal volume of water at 4°C

J.	Magnetism – ability of a mineral to be attracted to hand magnet
1.	Ferromagnetic – those that are attracted to hand magnet
2.	Paramagnetic – minerals that contain iron and ore attracted to hand magnet
3.	Diamagnetic – those that are repelled by a hand magnet

OTHER PROPERTIES OF MINERALS
A.	Tarnish – is the color of the thin surface of film of material which forms as a result of exposure to the atmosphere. i.e. Bornite has a bronze color on fresh surfaces but a purple tarnish which can be seen on weathered surfaces
B.	Play of Colors – the variation in colour of a mineral when rotated or tilted at different angles in height
1.	Opalescence – results from the regular arrangement of inclusions in the mineral
2.	Irridescence – includes labradorescence; is usually due to the refraction and/or reflection of light rays along cleavage, fractures, twin planes, exsolution lamellae, etc. e.g. Labradorite
C.	Chatoyancy – results from fibers or inclusions arranged in cavities
D.	Asterism – diffraction of light from regularly arranged inclusions, usually in tetragonal crystals
E.	Luminescence – ambient emission of light by a mineral due to the prscence of activators (impurities)
F.	Phosphorescence – a type of fluorescence that persists even after the “energy” or radiation source is removed
G.	Diaphaneity – is the ability of a mineral to pass light through it

Crystallography and Minerals Arranged by Crystal Form

•	Crystal Groups and Classes - Crystallography groups are composed of 32 classes of symmetry derived from observations of the external crystal form.  From these 32 classes, 230 space groups are distinguishable using x-ray analysis.  
•	Example Crystal Morphology - To further illustrate these symmetry elements, the example crystalline forms for each symmetry class were constructed using Faces (version 3.7)  The example crystals in each group and class were constructed using the following parameters:
•	Crystallographic axes: a=1, b=1, c=1. 
•	Forms: [214], [104], [024], [100], [010] (Color coded in all example crystals). 
This morphology was used in all the crystal groups and classes for drawing the examples and the stereo images. The higher order forms were selected to best illustrate some of the subtle differences in the symmetry classes.  Remember, most real crystals will probably not have similar forms when compared to the examples.
Table Crystallography (Mason 68, p18) - The thirty-two crystal classes are summarized by symmetry elements in the following table which is linked to: Example crystal images within the crystallography group in the yellow columns. Stereo crystal images/java example and mineral listings of each group and class in the green columns. 

The Cubic (Isometric) System 

Isometric
Crystallographic
Axes

Isometric Minerals Crystal Form Example 
[214], [104], [024], [100], [010]

Class Unknown Mineral Listing 

Isometric 
Diploidal Mineral Listing 
H-M Symbol (2/m 3) 

Isometric 
Gyroidal Mineral Listing 
H-M Symbol (4 3 2) 

Isometric 
Hexoctahedral Mineral Listing 
H-M Symbol (4/m 3 2/m) 

Isometric 
Hextetrahedral Mineral Listing 
H-M Symbol (4 3m) 

Isometric 
Tetartoidal Mineral Listing 
H-M Symbol (2 3) 

The Tetragonal System 

Tetragonal
Crystallographic
Axes

Tetragonal Mineral Crystal Form Example 
[214], [104], [024], [100], [010]

Class Unknown Mineral Listing 

Tetragonal Dipyramidal Mineral Listing 
H-M Symbol (4/m) 

Tetragonal Disphenoidal Mineral Listing 
H-M Symbol (4) 

Tetragonal Ditetragonal Dipyramidal Mineral Listing
 H-M Symbol (4/m 2/m 2/m) 

Tetragonal Pyramidal Mineral Listing 
H-M Symbol (4) 

Tetragonal Ditetragonal-pyramidal Mineral Listing 
H-M Symbol (4mm) 

Tetragonal Scalenohedral Mineral Listing 
H-M Symbol (4 2m) 

Tetragonal Trapezohedral Mineral 
H-M Symbol (4 2 2) 

The Orthorhombic System 

Orthorhobic
Crystallographic
Axes

Orthorhombic Minerals Crystal Form Example 
[214], [104], [024], [100], [010]

Class Unknown Mineral Listing. 

Orthorhombic 
Dipyramidal Mineral Listing 
H-M Symbol (2/m 2/m 2/m) 

Orthorhombic 
Disphenoidal Mineral Listing 
H-M Symbol (2 2 2) 

Orthorhombic 
Pyramidal Mineral Listing 
H-M Symbol (mm2) 

The Hexagonal System 
(Hexagonal Division) 

Hexagonal
Crystallographic
Axes

Hexagonal Minerals Crystal Form Example. 
[214], [104], [024], [100], [010]

Class Unknown Mineral Listing. 

Hexagonal 
Dihexagonal Dipyramidal Mineral Listing 
H-M Symbol (6/m 2/m 2/m) 

Hexagonal Dihexagonal Pyramidal Mineral Listing 
H-M Symbol (6mm) 

Hexagonal 
Dipyramidal Mineral Listing 
H-M Symbol (6/m) 

Hexagonal 
Ditrigonal Dipyramidal Mineral Listing 
H-M Symbol (6 m2) 

Hexagonal Pyramidal Mineral Listing 
H-M Symbol (6) 

Hexagonal 
Trapezohedral Mineral Listing 
H-M Symbol (6 2 2) 

Hexagonal 
Trigonal Dipyramidal Mineral 
H-M Symbol (6) 

The Hexagonal System 
(Trigonal Division) 

Trigonal 
Crystallographic 
Axes 

Trigonal Minerals Crystal Form Example. 
[214], [104], [024], [100], [010]

Class Unknown Mineral Listing. 

Trigonal 
Ditrigonal Pyramidal Mineral Listing and Stereo image. 
H-M Symbol (3m) 

Trigonal 
Hexagonal Scalenohedral Mineral Listing and Stereo image. 
H-M Symbol (3 2/m) 

Trigonal Pyramidal Mineral Listing and Stereo image. 
H-M Symbol (3) 

Trigonal 
Rhombohedral Mineral Listing and Stereo image. 
H-M Symbol ( 3) 

Trigonal Trapezohedral Mineral Listing and Stereo image. 
H-M Symbol (3 2) 

The Monoclinic System 

Monoclinic 
Crystallographic 
Axes 


Monoclinic Minerals Crystal Form Example. 
[214], [104], [024], [100], [010]

Class Unknown Mineral Listing. 

Monoclinic 
Domatic Mineral Listing 
H-M Symbol (m) 

Monoclinic 
Prismatic Mineral Listing 
H-M Symbol (2/m) 

Monoclinic 
Sphenoidal Mineral Listing 
H-M Symbol (2) 

The Triclinic System 

Triclinic 
Crystallographic 
Axes 


Triclinic Minerals Crystal Form Example. 
[214], [104], [024], [100], [010]

Class Unknown Mineral Listing. 

Triclinic 
Pedial Mineral Listing 
H-M Symbol (1) 

Triclinic 
Pinacoidal Mineral Listing 
H-M Symbol ( 1) 

 

Crystal Symmetries

•	An object or pattern has symmetry if it is seemingly unchanged by an operation such as rotation about an axis or reflection across a plane. If an object possesses rotational symmetry, it will appear unmoved after rotation about a rotation axis. For example, a hexagon looks unmoved after rotation of 60¡ about a perpendicular axis through its center, and a cube appears unmoved after rotation of 120¡ about an axis through a body diagonal. If a pattern possesses mirror symmetry, its two halves, separated by an imaginary plane, appear to be mirror images of one anotherÑfor example, the letter A. 

•	Crystallographers demonstrated that only certain angles of rotational symmetry are possible for crystals: 60¡, 90¡, 120¡, 180¡, and 360¡. The associated symmetry elements they termed 6-fold, 4-fold, 3-fold, 2-fold, and 1-fold rotation axes, respectively, based on the number of rotations required to equal 360¡. They further demonstrated that, for crystals, the possible combinations of symmetry operations involving rotation and reflection are limited. The combinations define 32 crystal classes, which are grouped into the 6 crystal systems: isometric, hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic. Further subdivision into the 230 space groups follows from the added consideration of "translation"Ñthat is, repetition in space by shifting, as in the building of a pattern by repeating a unit cell.

•	Crystals with well-developed faces may show some of the internal symmetry of the pattern. For example, crystals of beryl (BeOAlMSiRONT) commonly have 6 sides meeting at 120¡ interfacial angles, consistent with the 60¡ rotation axis of the beryl crystal structure. For crystals without good external form, physical tests involving light, electricity, force, sound, and so on can reveal some of their internal symmetry. If a crystal possesses rotational symmetry, the physical properties of the crystalÑsuch as the speed of sound or light within itÑwill be the same along directions that can be rotated into one another by a rotation axis. For example, simple optical tests using polarized light can help identify the crystal system of transparent crystals, based on the symmetry of the refraction (bending) of the light rays. Irregularly shaped crystals of galena (PbS) can be broken along 3 mutually perpendicular planes into tiny cubes, consistent with the isometric symmetry of the galena structure.

•	Knowledge of the space group symmetry and detailed structure of many crystals has come from more complicated physical tests involving X rays. Photons of electromagnetic radiationÑsuch as light or X raysÑare scattered by objects in their path. If the objects are part of a periodic array or pattern, coherent 

General Forms and Special Forms

•	A general form is a form in a particular crystal class that contains faces that intersect all crystallographic axes at different lengths. It has the form symbol {hkl}  All other forms that may be present are called special forms.  In the monoclinic, triclinic, and orthorhombic crystal systems, the form {111} is a general form because in these systems faces of this form will intersect the a, b, and c axes at different lengths because the unit lengths are different on each axis.  In crystals of higher symmetry, where two or more of the axes have equal length, a general form must intersect the equal length axes at different multiples of the unit length.  Thus in the tetragonal system the form {121} is a general form.  In the isometric system a general form would have to be something like {123}. 

Open Forms and Closed Forms

•	A closed form is a set of crystal faces that completely enclose space.  Thus, in crystal classes that contain closed forms, a crystal can be made up of a single form. 
•	An open form is one or more crystal faces that do not completely enclose space.  

Example 1.  Pedions are single faced forms.  Since there is only one face in the form a pedion cannot completely enclose space.  Thus, a crystal that has only pedions, must have at least 3 different pedions to completely enclose space.

Example 2.  A prism is a 3 or more faced form wherein the crystal faces are all parallel to the same line.  If the faces are all parallel then they cannot completely enclose space.  Thus crystals that have prisms must also have at least one additional form in order to completely enclose space.

Example 3.  A dipyramid has at least 6 faces that meet in points at opposite ends of the crystal.  These faces can completely enclose space, so a dipyramid is closed form.  Although a crystal may be made up of a single dipyramid form, it may also have other forms present. 

Pedions 
•	A pedion is an open, one faced form.  Pedions are the only forms that occur in the Pedial class (1).  Since a pedion is not related to any other face by symmetry, each form symbol refers to a single face.  For example the form {100} refers only to the face (100), and is different from the form {00} which refers only to the face (00). Note that while forms in the Pedial class are pedions, pedions may occur in other crystal classes. 

Pinacoids 
•	A Pinacoid is an open 2-faced form made up of two parallel faces. In the crystal drawing shown here the form {111} is a pinacoid and consists of two faces, (111) and ().  The form {100} is also a pinacoid consisting of the two faces (100) and (00).  Similarly the form {010} is a pinacoid consisting of the two faces (010) and (00), and the form {001} is a two faced form consisting of the faces (001) and (00).  In this case, note that at least three of the above forms are necessary to completely enclose space.  While all forms in the Pinacoid class are pinacoids, pinacoids may occur in other crystal classes as well. 

Domes 
•	Domes are 2- faced open forms where the 2 faces are related to one another by a mirror plane.  In the crystal model shown here, the dark shaded faces belong to a dome.  The vertical faces along the side of the model are pinacoids (2 parallel faces).  The faces on the front and back of the model are not related to each other by symmetry, and are thus two different pedions. 

Sphenoids 
•	Sphenoids are2 - faced open forms where the faces are related to each other by a 2-fold rotation axis and are not parallel to each other.  The dark shaded triangular faces on the model shown here belong  to a sphenoid.  Pairs of similar vertical faces that cut the edges of the drawing are also pinacoids.  The top and bottom faces, however, are two different pedions. 

Prisms 
•	A prism is an open form consisting of three or more parallel faces.  Depending on the symmetry, several different kinds of prisms are possible 

Trigonal prism
•	3 - faced form with all faces parallel to a 3 -fold rotation axis

Ditrigonal prism
•	6 - faced form with all 6 faces parallel to a 3-fold rotation axis.  Note that the cross section of this form (shown to the right of the drawing) is not a hexagon, i.e. it does not have 6-fold rotational symmetry 

Rhombic prism 
•	4 - faced form with all faces parallel to a line that is not a symmetry element.  In the drawing to the right, the 4 shaded faces belong to a rhombic prism.  The other faces in this model are pinacoids (the faces on the sides belong to a side pinacoid, and the faces on the top and bottom belong to a top/bottom pinacoid). 

Tetragonal prism 
•	4 - faced open form with all faces parallel to a 4-fold rotation axis or .  The 4 side faces in this model make up the tetragonal prism.  The top and bottom faces make up the a form called the top/bottom pinacoid. 

Ditetragonal prism 
•	8 - faced form with all faces parallel to a 4-fold rotation axis.  In the drawing, the 8 vertical faces make up the ditetragonal prism. 

Hexagonal prism 
•	6 - faced form with all faces parallel to a 6-fold rotation axis. The 6 vertical faces in the drawing make up the hexagonal prism.  Again the faces on top and bottom are the top/bottom pinacoid form 

Dihexagonal prism 
•	12 - faced form with all faces parallel to a 6-fold rotation axis. Note that a horizontal cross-section of this model would have apparent 12-fold rotation symmetry.  The dihexagonal prism is the result of mirror planes parallel to the 6-fold rotation axis. 

	

Pyramids 
•	A pyramid is a 3, 4, 6, 8 or 12  faced open form where all faces in the form meet, or could meet if extended, at a point. 

      Trigonal pyramid 
•	3-faced form where all faces are related by a 3-fold rotation axis. 

      Ditrigonal pyramid 
•	6-faced form where all faces are related by a 3-fold rotation axis. Note that if viewed from above, the ditrigonal pyramid would not have a hexagonal shape; its cross section would look more like that of the trigonal prism discussed above 

      Rhombic pyramid 
•	4-faced form where the faces are related by mirror planes. In the drawing shown here the faces labeled "p" are the four faces of the rhombic pyramid. If extend, these 4 faces would meet at a point. 

      Tetragonal pyramid 
•	4-faced form where the faces are related by a 4 axis. In the drawing the small triangular faces that cut the corners represent the tetragonal pyramid. Note that if extended, these 4 faces would meet at a point. 

      Ditetragonal pyramid 
•	8-faced form where all faces are related by a 4 axis.  In the drawing shown here, the upper 8 faces belong to the ditetragonal pyramid form.  Note that the vertical faces belong to the ditetragonal prism. 

      Hexagonal pyramid 
•	6-faced form where all faces are related by a 6 axis. If viewed from above, the hexagonal pyramid would have a hexagonal shape. 

      Dihexagonal pyramid 
•	12-faced form where all faces are related by a 6-fold axis. This form results from mirror planes that are parallel to the 6-fold axis. 

 Dipyramids 
•	Dipyramids are closed forms consisting of 6, 8, 12, 16, or 24 faces.  Dipyramids are pyramids that are reflected across a mirror plane.  Thus, they occur in crystal classes that have a mirror plane perpendicular to a rotation or rotoinversion axis.

      Trigonal dipyramid 
•	6-faced form with faces related by a 3-fold axis with a perpendicular mirror plane. In this drawing, all six faces belong to the trigonal-dipyramid 

      Ditrigonal -dipyramid 
•	12-faced form with faces related by a 3-fold axis with a perpendicular mirror plane. If viewed from above, the crystal will not have a hexagonal shape, rather it would appear similar to the horizontal cross-section of the ditrigonal prism, discussed above. 

      Rhombic dipyramid 
•	8-faced form with faces related by a combinations of 2-fold axes and mirror planes.  The drawing to the right shows 2 rhombic dipyramids.  One has the form symbol {111} and consists of the four larger faces shown plus four equivalent faces on the back of the model.  The other one has the form symbol {113} and consists of the 4 smaller faces shown plus the four on the back. 

      Tetragonal dipyramid 
•	8-faced form with faces related by a 4-fold axis with a perpendicular mirror plane.  The drawing shows the 8-faced tetragonal dipyramid.  Also shown is the 4-faced tetragonal prism, and the 2-faced top/bottom pinacoid. 

      Ditetragonal dipyramid 
•	16-faced form with faces related by a 4-fold axis with a perpendicular mirror plane. The ditetragonal dipyramid is shown here.  Note the vertical faces belong to a ditetragonal prism. 

      Hexagonal dipyramid 
•	12-faced form with faces related by a 6-fold axis with a perpendicular mirror plane.  The vertical faces in this model make up a hexagonal prism. 

      Dihexagonal dipyramid 
•	24-faced form with faces related by a 6-fold axis with a perpendicular mirror plane. 

Trapezohedrons 
•	Trapezohedron are closed 6, 8, or 12 faced forms, with 3, 4, or 6 upper faces offset from 3, 4, or 6 lower faces.  The trapezohedron results from 3-, 4-, or 6-fold axes combined with a perpendicular 2-fold axis.  

Scalenohedrons 
•	A scalenohedron is a closed form with 8 or 12 faces.  In ideally developed faces each of the faces is a scalene triangle.  In the model, note the presence of the 3-fold rotoinversion axis perpendicular to the 3 2-fold axes. 

Rhombohedrons 
•	A rhombohedron is 6-faced closed form wherein 3 faces on top are offset by 3 identical upside down faces on the bottom, as a result of a 3-fold rotoinversion axis.  Rhombohedrons can also result from a 3-fold axis with perpendicular 2-fold axes.  Rhombohedrons only occur in the crystal classes 2/m , 32, and . 

Disphenoids 
•	A disphenoid is a closed form consisting of 4 faces.  These are only present in the orthorhombic system (class 222) and the tetragonal system (class ) 

Hexahedron 
•	A hexahedron is the same as a cube.  3-fold axes are perpendicular to the face of the cube, and four axes run through the corners of the cube. Note that the form symbol for a hexahedron is {100}, and it consists of the following 6 faces:  (100), (010), (001), (00), (00), and (00). 

Octahedron 
•	An octahedron is an 8 faced form that results form three 4-fold axes with perpendicular mirror planes.  The octahedron has the form symbol {111}and consists of the following 8 faces: (111),  (), (11), (1), (1), (1), (11), and (11).  Note that four 3-fold axes are present that are perpendicular to the triangular faces of the octahedron (these 3-fold axes are not shown in the drawing).

Dodecahedron 
•	A dodecahedron is a closed 12-faced form.  Dodecahedrons can be formed by cutting off the edges of a cube.  The form symbol for a dodecahedron is {110}.  As an exercise, you figure out the Miller Indices for these 12 faces.

Tetrahexahedron 
•	The tetrahexahedron is a 24-faced form with a general form symbol of {0hl} This means that all faces are parallel to one of the a axes, and intersect the other 2 axes at different lengths.

Trapezohedron 
•	An isometric trapezohedron is a 12-faced closed form with the general form symbol {hhl}.  This means that all faces intersect two of the a axes at equal length and intersect the third a axis at a different length. 

Tetrahedron 
•	The tetrahedron occurs in the class 3m and has the form symbol {111}(the form shown in the drawing) or {11} (2 different forms are possible).  It is a four faced form that results form three axes and four 3-fold axes (not shown in the drawing).

Gyroid 
•	A gyroid is a form in the class 432 (note no mirror planes) 

Pyritohedron 
•	The pyritohedron is a 12-faced form that occurs in the crystal class 2/m.  Note that there are no 4-fold axes in this class.  The possible forms are {h0l} or {0kl} and each of the faces that make up the form have 5 sides.

Diploid 
•	The diploid is the general form {hkl} for the diploidal class (2/m).  Again there are no 4-fold axes. 

Tetartoid 
•	Tetartoids are general forms in the tetartoidal class (23) which only has 3-fold axes and 2-fold axes with no mirror planes.  

Vectorial Properties of Crystals

Continuous Vectorial Properties
-	Continuous vectorial properties depend on direction, but along any given the direction the property is the same.  Some of the continuous vectorial properties are:

•	Hardness - In some minerals there is a difference in hardness in different directions in the crystal. Examples: Kyanite, Biotite, Muscovite.  This can become an important identifying property and/or may lead to confusion about the hardness if one is not aware of the directional dependence.

•	Velocity of Light (Refractive Index) - For all minerals except those in the isometric system, the velocity of light is different as the light travels along different directions in the crystal.  We will use this directional dependence of light velocity as an important tool in the second half of the course.  Refractive Index is defined as the velocity of light in a material divided by the velocity of light in a vacuum.  Because the velocity of light depends on direction, the refractive index will also depend on direction.

•	Thermal Conductivity - The ability of a material to conduct heat is called thermal conductivity.  Like light, heat can be conducted at different rates along different directions in crystals.

•	Electrical Conductivity-  The ability of a material to allow the passage of electrons is called electrical conductivity, which is also directionally dependent except in isometric crystals.

•	Thermal Expansion - How much the crystal lattice expands as it is heated is referred to as thermal expansion.  Some crystals expand more in one direction than in others, thus thermal expansion is a vectorial property.

•	Compressibility - Compressibility is a measure of how the lattice is reduced as atoms are pushed closer together under pressure.  Some directions in crystals may be more compressible than others.

Discontinuous Vectorial Properties
-	Discontinuous vectorial properties pertain only to certain directions or planes within a crystal.  For these kinds of properties, intermediate directions may have no value of the property.  Among the discontinuous vectorial properties are:

•	Cleavage -  Cleavage is defined as a plane within the lattice along which breakage occurs more easily than along other directions.   A cleavage direction develops along zones of weakness in the crystal lattice.  Cleavage is discontinuous because it only occurs along certain planes.

•	Growth Rate - Growth rate is defined as the rate at which atoms can be added to the crystal. In some directions fewer atoms must be added to the crystal than in other directions, and thus some directions may allow for faster growth than others. 

•	Solution Rate - Solution rate is the rate at which a solid can be dissolved in a solvent.  In this case it depends on how tightly bonded the atoms are in the crystal structure, and this usually depends on direction. 

Crystal Habit

•	In nature perfect crystals are rare.  The faces that develop on a crystal depend on the space available for the crystals to grow.  If crystals grow into one another or in a restricted environment, it is possible that no well-formed crystal faces will be developed.  However, crystals sometimes develop certain forms more commonly than others, although the symmetry may not be readily apparent from these common forms.  The term used to describe general shape of a crystal is habit.

•	Some common crystal habits are as follows.
Cubic - cube shapes
Octahedral - shaped like octahedrons, as described above.
Tabular -  rectangular shapes.
Equant - a term used to describe minerals that have all of their boundaries of approximately equal length.

The Inosilicate

•	This subclass contains two distinct groups: the single chain and double chain silicates. In the single chain group the tetrahedrons share two oxygens with two other tetrahedrons and form a seemingly endless chain. The ratio of silicon to oxygen is thus 1:3. The tetrahedrons alternate to the left and then to the right along the line formed by the linked oxygens although more complex chains seem to spiral. In cross section the chain forms a trapezium and this shape produces the angles between the crystal faces and cleavage directions. 

•	In the double chain group, two single chains lie side by side so that all the right sided tetrahedrons of the left chain are linked by an oxygen to the left sided tetrahedrons of the right chain. The extra shared oxygen for every four silicons reduces the ratio of silicons to oxygen to 4:11. The double chain looks like a chain of six sided rings that might remind someone of a child's clover chain. The cross section is similar in the double chains to that of the single chains except the trapezium are longer in the double chains. This difference produces a difference in angles. The cleavage of the two groups results between chains and does not break the chains thus producing prismatic cleavage. In the single chained silicates the two directions of cleavage are at nearly right angles (close to 90 degrees) forming nearly square cross sections. In the double chain silicates the cleavage angle is close to 120 and 60 degrees forming rhombic cross sections making a convenient way to distinguish double chain silicates from single chain silicates. 

Single Chain Inosilicates
THE MINERAL LORENZENITE

•	Chemistry: Na2Ti2Si2O9, Sodium Titanium Silicate. 
•	Class: Silicates 
•	Subclass: Inosilicates 
•	Uses: mineral specimens 
•	Specimens 
•	Lorenzenite is a rare mineral that was only discribed in the last 50 years. It is found on both the coast of Greenland and on the Kola Pennisula from where many new and rare minerals have been and are still being discovered. Ramsayite is the old russian name for lorezenite. Now the name Ramsayite is a synonym for lorenzenite. Lorenzenite has a rather high luster which is due to the titanium content. Other titanium minerals also have high lusters especially the mineral rutile, TiO2. 

PHYSICAL CHARACTERISTICS:
•	Color is a brown, yellow-brown to dark brown. 
•	Luster is vitreous to submetallic. 
•	Transparency: Crystals are transparent to translucent. 
•	Crystal System is Orthorhombic. 
•	Crystal Habit is prismatic with six sides, two of the sides are often flattened to form a nearly tabular crystal with a pointed termination. 
•	Hardness is 6. 
•	Specific Gravity is approximately 3.4+ (above average for translucent minerals)
•	Streak is light brown. 
•	Notable Occurrences: Narsarsuk, Greenland and Kola Pennisula, Russia.
•	Best Field Indicators are color, luster, density and locallity. 

The Mineral NEPTUNITE

•	Chemistry: KNa2Li(Fe, Mn)2Ti2Si8O24, Potasium Sodium Lithium Iron Manganese Titanium Silicate 
•	Class: Silicates 
•	Subclass: Inosilicates 
•	Uses: mineral specimens. 
•	Specimens 
•	Neptunite was only discovered during this century and is of rare occurence. Its most famous specimens come from the wonderful deposits at San Benito County in California. This site contains natrolite veins in a serpentinite rock. Embedded in the pure white natrolite are the sapphire blue benitoite and the black prismatic crystals of neptunite. The assemblage is a classic among collectors and specimens with all three minerals are much sought after. Neptunite is named after the Roman god of the sea, Neptune. Crystals are well formed and typically show multiple secondary faces. The flashes of red that appear to come right from the center of the crystal are a nice added touch to this rare and interesting mineral. 

PHYSICAL CHARACTERISTICS:
•	Color is black to reddish black. 
•	Luster is vitreous. 
•	Transparency crystals are translucent to opaque. 
•	Crystal System is monoclinic; 2/m 
•	Crystal Habits include prismatic, elongated crystals with nearly square cross-sections. The terminations are modified and slanted dome faces. Also found massive and in disseminated grains. 
•	Cleavage is perfect in two direction at 80 and 100 degree angles. 
•	Fracture is splintery. 
•	Hardness is 5.5 
•	Specific Gravity is approximately 3.23 (slightly above average) 
•	Streak is reddish-brown. 
•	Associated Minerals are benitoite, natrolite, serpentine, aegerine and joaquinite. 
•	Other Characteristics: an internal red reflection can sometimes be seen through the prismatic crystal faces. 
•	Notable Occurrences include the famous San Benito Co., California site as well as Narsarssuk Greenland and the Kola penninsula, Russia. 
•	Best Field Indicators are crystal habit, internal reflection, streak, associations, color, and cleavage 

Double Chain Inosilicates

•	The amphibole minerals are inosilicates of the general formula XY2Z5(Si, Al, Ti)8O22(OH, F)2. The X represents large ions such as sodium or potassium and this site can be left vacant. The Y can be populated by sodium, calcium, iron (+2), lithium, manganese (+2), aluminum and/or magnesium and more rarely zinc, nickel, or cobalt. The Z can be filled by ions such as Iron (+3), manganese (+3), chromium (+3), aluminum, titanium (+4), iron (+2), lithium and manganese (+2). Most amphiboles are monoclinic, 2/m; but members of the Anthophyllite Subgroup are orthorhombic, 2/m 2/m 2/m. The rather complex possibilities of formulas does lead to a lot of variety in the amphibole group. Compare the formulas of these common amphiboles:

THE MINERAL HORNBLENDE

•	Chemistry: Ca2(Mg, Fe, Al)5 (Al, Si)8O22(OH)2, Calcium Magnesium Iron Aluminum Silicate Hydroxide. 
•	Class: Silicates 
•	Subclass: Inosilicates 
•	Group: Amphibole 
•	Uses: Only as mineral specimens 
•	Specimens 
•	Hornblende is actually the name given to a series of minerals that are rather difficult to distinguish by ordinary means. The iron, magnesium and aluminum ions can freely substitute for each other and form what have been distinguished as seperate minerals. The minerals are given the names Magnesio-hornblende, Ferrohornblende, Alumino-ferro-hornblende and Alumino-magnesio-hornblende. These minerals are obviously named for their chemistries although there is little to distinguish them in the field. The iron rich members of the series are more darker black and less likely to be translucent. 

•	Hornblende is not often a collection mineral because good crystals are somewhat difficult to find even though the mineral is widespread. It is almost always opaque and black and not very attractive. However a few specimens are extraordinary and make for valuable specimens. Some crystals can grow to a fairly large size of several feet long and nearly a foot across. Other specimens of hornblende can be acicular clusters or needle thin crystal aggregates. Many times a specimen of a more valuable mineral will be accented by the opaque black crystals of hornblende. 

PHYSICAL CHARACTERISTICS:
•	Color is almost always black to dark green. 
•	Luster is vitreous to dull. 
•	Transparency: Crystals are generally opaque but thin crystals or exceptional specimens can be translucent. 
•	Crystal System is Monoclinic; 2/m 
•	Crystal Habits include short stocky prismatic crystals as well as long thin crystal forms. Crystals can have a hexagonal cross-section although rarely symmetrical. The typical termination, if seen, appears as the two faces of a dome but is actually two of the four faces of a prism. Also found granular, massive and occassionally acicular aggregates. 
•	Cleavage is imperfect in two directions at 56 and 124 degrees. 
•	Fracture is uneven. 
•	Hardness is 5 - 6. 
•	Specific Gravity is approximately 2.9 - 3.4 (somewhat above average for translucent minerals) 
•	Streak is brown to gray. 
•	Other Characteristics: pleochroic in translucent speciments. Large crystals have an almost striated or grainy appearance. 
•	Associated Minerals are quartz, feldspars, augite, magnetite, micas and many medium grade metamorphic minerals. 
•	Notable Occurrences are numerous and include Bancroft, Ontario; Norway; Bohemia; Mt. Vesuvius, Italy and New York, USA. 
•	Best Field Indicators are crystal habit (especially cross-section), color and cleavage. 

THE MINERAL TREMOLITE

•	Chemistry: Ca2Mg5Si8O22(OH)2 , Calcium Magnesium Silicate Hydroxide. 
•	Class: Silicates 
•	Subclass: Inosilicates 
•	Group: Amphibole 
•	Uses: Asbestos and as a mineral specimen. 
•	Specimens 
•	Tremolite is a relatively common mineral in some metamorphic rocks. It occurs from the conversion of dolomite, silica and water into tremolite, calcite and carbon dioxide by way of the following formula: 

5CaMg(CO3)2 + 8SiO2 + H2O ------> Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 7CO2 

•	It belongs to a series with the minerals actinolite and ferro-actinolite. A series occurs when two or more ions can freely substitute between each other. In this case, when iron is predominant the mineral is ferro-actinolite and when magnesium is predominant the mineral is tremolite. Actinolite is the intermediate member and the most common followed by tremolite. Tremolite's formula is often written as the same as actinolite (with iron and magnesium), but specimens of tremolite can be found quite pure (that is, free of iron) and so here the formula reflects the pure end of the series. The entire series can be represented with the actinolite formula: Ca2(Mg, Fe)5Si8O22(OH)2. 

•	Without the iron in the formula tremolite will have the typical creamy white color. With just a small amount of iron tremolite will be green. Increasing iron content will raise the specific gravity, index of refraction and darken the color. 

PHYSICAL CHARACTERISTICS:
•	Color is usually white or gray but can be greenish, colorless, yellow and violet.
•	Luster is vitreous or silky to dull. 
•	Transparency: Specimens are translucent to transparent. 
•	Crystal System is monoclinic; 2/m 
•	Crystal Habits include flattened prismatic and elongated crystals with a dome-like termination that is actually two of the four faces of a prism. Fibrous crystals form radial aggregates, masses and hair like clusters. Also as a felted mass (asbestos and "mountain leather"). 
•	Cleavage: is perfect in two directions at close to 60- and 120-degree angles (diamond-shaped). 
•	Fracture is uneven. 
•	Hardness is 5 - 6. 
•	Specific Gravity is approximately 2.9 - 3.1 (very slightly above average for translucent minerals). 
•	Streak is white. 
•	Other Characteristics: Index of refraction is 1.60 - 1.63. 
•	Associated Minerals are calcite, grossular, talc and serpentine. 

THE MINERAL FLUORRICHTERITE

•	Chemistry: Na2Ca(Mg, Fe)5Si8O22(OH, F)2, Sodium Calcium Magnesium Iron Silicate Hydroxide Fluoride. 
•	Class: Silicates 
•	Subclass: Inosilicates 
•	Group: Amphibole 
•	Uses: Only as mineral specimens. 
•	Specimens 
•	Fluorrichterite is a rare mineral, known from only one location. It is found near Wilberforce, Ontario in a skarn formation. The skarn is a term for a rock that is the product of a chemically unusual magma body that has intruded into and recrystallized a "dirty" limestone. The "dirty" limestone is not composed of just calcite, CaCO3, like "clean" limestone but is mixed with silicates and/or phosphates, etc. This mixing of the hot chemically unusual liquids and volatiles of the magma with the different minerals of the "dirty" limestones produces some interesting and rare minerals after all the recrystallizing is done. One of these is of course, fluorrichterite. 

•	Fluorrichterite forms well shaped crystals that can have a pearly luster. Its luster is higher than most members of the amphibole group possibly because of the unusual presence of fluorine which comes from the unusual magma chemistry. Mixed with fluorrichterite in the white recrystallized calcite of the skarn are well formed crystals of biotite making specimens extra special. 

PHYSICAL CHARACTERISTICS:
•	Color is dark green to blue green or less commonly brown, gray and black. 
•	Luster is vitreous to pearly to dull. 
•	Transparency: Crystals are generally translucent to opaque. 
•	Crystal System is Monoclinic; 2/m 
•	Crystal Habits include prismatic crystals with an overall nearly diamond shaped cross-section with the four points usually cut by minor prism faces. Due to dissolution reactions after initial crystallization, the crystal faces can appear pitted and the terminations can be rounded. Twinning is commonly seen and results in a groove or notch running down the "spine" of the prismatic crystals. 
•	Cleavage is imperfect in two directions at nearly 60 and 120 degrees. 
•	Fracture is uneven. 
•	Hardness is 6. 
•	Specific Gravity is approximately 3 (average for translucent minerals) 
•	Streak is white. 
•	Other Characteristics: Crystals can have a iridescent shimer and associated minerals can be seen intergrown in some of the crystals filling pits and voids on their surface. 
•	Associated Minerals are biotite, molybdenite, cancrinite and calcite 
•	Notable Occurrence is several sites near Wilberforce, Ontario, Canada. 

Nesosilicate (single tetrahedrons)

•	The simplest of all the silicate subclasses, this subclass includes all silicates where the (SiO4) tetrahedrons are unbonded to other tetrahedrons. In this respect they are similar to other mineral classes such as the sulfates and phosphates. These other classes also have tetrahedral basic ionic units (PO4 & SO4) and thus there are several groups and minerals within them that are similar to the members of the nesosilicates. Nesosilicates, which are sometimes referred to as orthosilicates, have a structure that produces stronger bonds and a closer packing of ions and therefore a higher density, index of refraction and hardness than chemically similar silicates in other subclasses. Consequently, There are more gemstones in the nesosilicates than in any other silicate subclass. Below are the more common members of the nesosilicates. See the nesosilicates' page for a more complete list.

THE MINERAL OLIVINE

•	Chemistry: (Mg, Fe)2SiO4, Magnesium Iron Silicate. 
•	Class: Silicates 
•	Subclass: Nesosilicates 
•	Group: Olivine 
•	Uses: As gemstones, industrial uses as refractory sands and abrasives, an ore of magnesium and as mineral specimens. 
•	Specimens 
•	Also see variety specimens: 
	-Peridot Specimens 
•	Olivine is actually a name for a series between two end members, fayalite and forsterite. Fayalite is the iron rich member with a pure formula of Fe2SiO4. Forsterite is the magnesium rich member with a pure formula of Mg2SiO4. The two minerals form a series where the iron and magnesium are substituted for each other without much effect on the crystal structure. Fayalite due to its iron content has a higher index of refraction, is heavier and has a darker color than forsterite. Otherwise they are difficult to distinguish and virtually all specimens of the two minerals contain both iron and magnesium. For simplicity sake and general public recognition, they are often treated as one mineral, olivine. Olivine, however is not officially recognized as a mineral (see other non-minerals such as apophyllite, tourmaline, mica, serpentine, chlorite and apatite). 

PHYSICAL CHARACTERISTICS:
•	Color is a light near emerald green to the more common pale yellowish green; also found colorless, greenish brown to black. 
•	Luster is vitreous. 
•	Transparency: Crystals are transparent to translucent. 
•	Crystal System is orthorhombic; 2/m 2/m 2/m. 
•	Crystal Habits include flatten tabular to box shaped crystals, but good crystals are rare. More commonly found as grains in alluvial gravels and as granular xenoliths in magnesium rich volcanic rock. Also massive. Twinning is rare, but has produced star shaped trillings. 
•	Cleavage is poor in two directions at 90 degrees, is more distinct in fayalite. 
•	Fracture is conchoidal. 
•	Hardness is 6.5 - 7. 
•	Specific Gravity is approximately 3.2 for forsterite - 4.3 for fayalite (above average for non-metallic minerals). 
•	Streak is white. 
•	Other Characteristics: Index of refraction is 1.64 - 1.70 and has double refraction. 
•	Associated Minerals are diopside, spinel, plagioclase feldspars, chromite, hornblende, serpentine, iron-nickel meteorites and augite. 

THE MINERAL EUCLASE

•	Chemistry: BeAlSiO4OH, Beryllium Aluminum Silicate Hydroxide. 
•	Class: Silicates 
•	Subclass: Nesosilicates 
•	Uses: Gemstone and as mineral specimens. 
•	Specimens 
•	Euclase is not a well known gemstone, but is more well known by mineral collectors. It forms well formed crystals that occasionally have enough clarity to be cut as gems. The sapphire blue and blue green colors of euclase are the typical colors of the gemstones. 

•	The crystals of euclase are commonly prismatic with slanted termination faces. Unlike other monoclinic minerals whose crystals will commonly look like higher symmetry crystals, euclase's crystals are distinctly monoclinic due to the slanted termination. Nice crystals of euclase are much sought after by collectors. 

•	Euclase is found in granitic pegmatites with other gem minerals such as topaz and beryl. It is easily identified by its crystal form which can only be confused with barite or celestite but is the wrong environment for these sulfate minerals. Topaz has a completely different cleavage (basal) than euclase (prismatic). Transparent albite is softer. Euclase weathered out of source rock and transported down stream can end up being found in placer deposits with gold. 

PHYSICAL CHARACTERISTICS:
•	Color includes colorless, blue-green, blue, yellow, light green and rarely purple.
•	Luster is vitreous. 
•	Transparency: Crystals are transparent to translucent. 
•	Crystal System is monoclinic; 2/m 
•	Crystal Habits include stubby prismatic crystals with non-symmetrical slanted terminations. 
•	Cleavage is perfect in one direction (prismatic), poor in two other directions. 
•	Fracture is conchoidal. 
•	Hardness is 7.5 
•	Specific Gravity is 3.09 - 3.11 (slightly above average) 
•	Streak is white. 
•	Other Characteristics: Almost always striated lengthwise. 
•	Associated Minerals include micas, quartz, topaz, beryl, gold, pericline and some feldspars. 
•	Notable Occurrences include Ouro Preto and other areas within Minas Gerias, Brazil; Kenya; Tanzania; Ural Mountains, Russia; Germany and with emeralds in the famous mines of Columbia. 
•	Best Field Indicators are crystal habit, good cleavage, striations, hardness and color. 

Sorosilicate (double tetrahedrons)

•	Sorosilicates have two silicate tetrahedrons that are linked by one oxygen ion and thus the basic chemical unit is the anion group (Si2O7) with a negative six charge (-6). This structure forms an unusual hourglass-like shape and it may be due to this oddball structure that this subclass is the smallest of the silicate subclasses. It includes minerals that may also contain normal silicate tetrahedrons as well as the double tetrahedrons. The more complex members of this group, such as Epidote, contain chains of aluminum oxide tetrahedrons being held together by the individual silicate tetrahedrons and double tetrahedrons. Most members of this group are rare, but epidote is widespread in many metamorphic environments. 

THE MINERAL LEUCITE

•	Chemistry: KAlSi2O6, Potassium Aluminum Silicate 
•	Class: Silicates 
•	Subclass: Tektosilicates 
•	Group: Feldspathoids 
•	Uses: mineral specimen and as a minor source of potassium and aluminum. 
•	Specimens 
•	Leucite is a popular and interesting mineral. Its name comes from the greek word for "white" in allusion to its typical color. At high temperatures, leucite is isometric and will form the isometric trapezohedron crystal form. Interestingly, as leucite cools, the isometric structure becomes unstable and transforms into a tetragonal structure without altering the outward shape. Although the mineral is actually tetragonal, the outward shape is pseudo-isometric and thus the crystal form is actually pseudo-trapezohedral. 
•	Leucite is one of the few minerals that forms the unique trapezohedron. The trapezohedron has 24 deltoid shaped faces, where each face occupies one third of the position of a single octahedron's face. The minerals of the garnet group and the mineral analcime are the only common minerals that will also form the trapezohedron. 

•	Distinguishing leucite from the garnets and analcime is relatively easy in some cases. The garnets are much harder and usually deeply colored. Leucite has a much lower density and usually has a duller luster than analcime. Also leucite is typically embedded in host rock where as analcime, when displaying good crystals and not massive or granular, is loose or attacted to other minerals in volcanic cavities. 

•	Leucite, KAlSi2O6 is actually distantly related to analcime, NaAlSi2O6-H2O. Leucite is a member of the feldspathoid group of minerals. Analcime, although usually considered a zeolite, is sometimes placed in the feldspathoid group since its chemistry and occassional occurrences are similar. 

PHYSICAL CHARACTERISTICS:
•	Color is clear, white or gray, with yellowish and reddish tints possible. 
•	Luster is vitreous or greasy to dull. 
•	Transparency: crystals are transparent, translucent to commonly opaque. 
•	Crystal System is isometric; 4/m bar 3 2/m at temperatures above 605 degrees C, and tetragonal; either 4 2 2 or 4/m, below 605 degrees C. 
•	Crystal Habits include the characteristic trapezohedron (actually pseudo-trapezohedron). Also granular and massive. 
•	Cleavage is absent. 
•	Fracture is conchoidal 
•	Hardness is 5.5 - 6. 
•	Specific Gravity is approximately 2.4 - 2.5 (slightly below average) 
•	Streak is white. 
•	Associated Minerals include olivine, labradorite, augite, biotite, nepheline and other feldspathoids. 
•	Other Characteristics: surface is usually pitted, dull or has a weathered look. 
•	Notable Occurrences include Mt. Vesuvius, Italy; Magnet Cove, Arkansas, Leucite Hills, Wyoming and Litchfield, Maine, USA; Brazil and Hastings Co, Ontario. 
•	Best Field Indicators are crystal habit, density, low hardness, luster and associations. 

THE MINERAL DANBURITE

•	Chemistry: CaB 2 Si 2 O 8 , Calcium boron silicate 
•	Class: Silicates 
•	Subclass: Sorosilicates 
•	Uses: mineral specimen 
•	Specimens 
•	Danburite is not a well-known mineral but is growing in popularity. With crystals similar to topaz, danburite offers a unique opportunity for a collector who likes perfectly transparent crystal clusters. People who admire clear quartz will like the different look of danburite. Its diamond-shaped cross-section and wedge like termination is a contrast to quartz's hexagonal prisms and pyramidal terminations. Danburite's original locality is now buried under the city of Danbury, Connecticut. 

PHYSICAL CHARACTERISTICS:
•	Color is almost always clear or white, but also can be shades of yellow, pink and brown. 
•	Luster is vitreous. 
•	Transparency: Crystals are transparent to translucent. 
•	Crystal System is orthorhombic; 2/m 2/m 2/m 
•	Crystal Habits include generally prismatic crystals with a diamond-shaped cross-section. The termination is a steeply slanted dome producing a wedge look. Crystals can be rather thin to almost acicular or up to a foot long and several inches across. 
•	Cleavage is poor in one direction, basal. 
•	Fracture is uneven to conchoidal. 
•	Hardness is 7 - 7.5 
•	Specific Gravity is approximately 3.0 (average) 
•	Streak is white or colorless. 
•	Associated Minerals are quartz, feldspars, cassiterite, dolomite, corundum variety ruby, and fluorite. 
•	Notable Occurrences include Danbury, Connecticut and Russell, New York, USA; Charcas, San Luis Potosi, Mexico; Kyushu Is., Japan; Mogok, Burma and Uri, Switzerland. 
•	Best Field Indicators are crystal habit, poor cleavage, hardness and associations. 

Cyclosilicate (rings)

•	These silicates form chains such as in the inosilicates except that the chains link back around on themselves to form rings. The silicon to oxygen ratio is generally the same as the inosilicates, (1:3). The rings can be made of the minimum three tetrahedrons forming triangular rings (such as in benitoite). Four tetrahedrons can form a rough square shape (such as in axinite). Six tetrahedons form hexagonal shapes (such as in beryl, cordierite and the tourmalines). There are even eight membered rings and more complicated ring structures. The symmetry of the rings usually translates directly to the symmetry of these minerals; at least in the less complex cyclosilicates. Benitoite's ring is a triangle and the symmetry is trigonal or three-fold. Beryl's rings form hexagons and its symmetry is hexagonal or six-fold. The Tourmalines' six membered rings have alternating tetrahedrons pointing up then down producing a trigonal as opposed to an hexagonal symmetry. Axinite's almost total lack of symmetry is due to the complex arrangement of its square rings, triangle shaped borate anions (BO3) and the position of OH groups. Cordierite is pseudo-hexagonal and is analogous to beryl's structure except that aluminums substitute for the silicons in two of the six tetrahedrons. There are several gemstone minerals represented in this group, a testament to the general high hardness, luster and durability. Below are the more common members of the cyclosilicates. See the Cyclosilicates' page for a more complete list.

THE MINERAL AXINITE

•	Chemistry: Ca2(Mn, Fe, Mg)Al2(BO3OH)(SiO3)4, Calcium Manganese Iron Magnesium Aluminum Borosilicate Hydroxide 
•	Class: Silicates 
•	Subclass: Cyclosilicates 
•	Uses: Only as a mineral specimen 
•	Specimens 
•	Axinite is an interesting mineral and produces unique flattened spatula shaped crystals that fanciers of well formed crystals love to collect. It is a triclinic mineral which means that it has no symmetry planes or axes of rotation. It does possess a center of symmetry. A center of symmetry, or center, is a symmetry operation that takes a face on one side of a crystal and inverts in through the center of the crystal to the other side. The faces that are inverted are completely flipped both left to right and up to down. With this being the only symmetry operation, faces on axinite crystals are seemingly scattered everywhere and anywhere (except for their inverted partners on the other side of the crystal). 
•	Ferro-axinite, iron rich, lilac brown to black, SG=3.31 
•	Magnesio-axinite, magnesium rich, pale blue to gray, SG=3.18 
•	Manganaxinite, manganese rich, yellow-orange, SG=3.32 
•	Tinzenite, iron & manganese intermediate, yellow, SG=3.37 

PHYSICAL CHARACTERISTICS:
•	Color is commonly a lilac brown but also yellow, yellow-orange, gray, pale blue and even black. 
•	Luster is vitreous. 
•	Transparency crystals are transparent to translucent. 
•	Crystal System is triclinic; bar 1 
•	Crystal Habits include flattened wedge shaped crystals, often with a spatula or knife-like shaped edge. Also as granular and as parallel bladed aggregates. 
•	Cleavage is good in one direction. 
•	Fracture is conchoidal. 
•	Hardness is 6 - 7.5 
•	Specific Gravity is approximately 3.0 - 3.4 (slightly above average for transparent minerals) 
•	Streak is white. 
•	Other Characteristics: crystals are heavily striated on some faces and manganese rich axinites have been known to be fluorescent red. 
•	Associated Minerals are diopside, andradite, quartz, calcite, epidote, scheelite and prehnite. 
•	Notable Occurrences include Madera Co., California and Franklin, New Jersey, USA; Baja California, Mexico; Bahia, Brazil; Switzerland; Obira, Japan; Cornwall, England and France. 
•	Best Field Indicators are crystal habit, hardness, color and striations. 

THE MINERAL BARATOVITE

•	Chemistry: KLi3Ca7(Ti, Zr)2(SiO3)12F2, Potassium Lithium Calcium Titanium Zirconium Silicate Fluoride. 
•	Class: Silicates 
•	Subclass: cyclosilicates 
•	Uses: only as a mineral specimen 
•	Specimens 
•	Baratovite is a very rare silicate mineral. It is extremely new on the mineral scene since it was only described in the last twenty five years. Baratovite is a cyclosilicate which means that the structure is composed of silicate tetrahedrons linked together so that they form isolated rings. Baratovite is one of the rare minerals that are coming out of the former USSR and finding their way onto the mineral markets. Baratovite has the "look" of muscovite but is easily distinguished by its brilliant blue fluorescence under ultra-violet light. 

PHYSICAL CHARACTERISTICS:
•	Color is white with pinkish tints. 
•	Luster is pearly. 
•	Transparency crystals are transparent to translucent. 
•	Crystal System is monoclinic (pseudo-hexagonal); 2/m 
•	Crystal Habits include flattened pseudohexagonal crystals also massive and lamellar. 
•	Cleavage: perfect in one direction. 
•	Fracture: uneven 
•	Hardness is 3.5 
•	Specific Gravity is approximately 2.9 (average) 
•	Streak is white. 
•	Other Characteristics: fluoresces brilliant blue under UV light. 
•	Associated Minerals include aegerine, misterite and other rare silicates. 
•	Notable Occurrences include Dara-Pioz, Tien-Shan Mountains, Tadzhikistan. 
•	Best Field Indicators are crystal habit, fluorescence, cleavage, associations and locality. 

Phyllosilicate (sheets)

•	In this subclass, rings of tetrahedrons are linked by shared oxygens to other rings in a two dimensional plane that produces a sheet-like structure. The silicon to oxygen ratio is generally 1:2.5 (or 2:5) because only one oxygen is exclusively bonded to the silicon and the other three are half shared (1.5) to other silicons. The symmetry of the members of this group is controlled chiefly by the symmetry of the rings but is usually altered to a lower symmetry by other ions and other layers. The typical crystal habit of this subclass is therefore flat, platy, book-like and display good basal cleavage. Typically, the sheets are then connected to each other by layers of cations. These cation layers are weakly bonded and often have water molecules and other neutral atoms or molecules trapped between the sheets. This explains why this subclass produces very soft minerals such as talc, which is used in talcum powder. Some members of this subclass have the sheets rolled into tubes that produce fibers as in asbestos serpentine. 

THE MINERAL APOPHYLLITE

•	Chemistry: (K,Na)Ca4Si8O20(F,OH) - 8H2O, Hydrated Potassium Calcium Sodium Silicate Fluoride Hydroxide. 
•	Class: Silicates 
•	Subclass: Phyllosilicates 
•	Group: Apophyllite 
•	Uses: Only as mineral specimens. 
•	Physical Properties 
•	Specimens 
•	Apophyllite, whose name roughly means "to leaf apart" in Greek, is a mineral classic. It was given its name because crystals tend to peel or flake apart when they are heated due to the loss of water molecules. Although not that well-known by the general public, apophyllite is quite popular among mineral collectors. It is probably the first exotic mineral that a young collector will own after filling up on specimens of calcite, quartz, pyrite, galena, mica, fluorite, gypsum, apatite, etc, etc. After these common minerals, apophyllite seems like a real rarity and it offers so much to the collector. It has beauty, pastel colors, a bright luster, interesting well formed habits, unusual associations with other exotic minerals and recently large amounts of quality specimens have become available at amazingly low prices from just a decade ago 

THE PHYSICAL CHARACTERISTICS:
•	Color is clear, white, green, yellow, pink, violet or rarely brown. 
•	Luster is vitreous to pearly on cleavage surfaces. 
•	Transparency: Crystals are transparent to translucent. 
•	Crystal System is tetragonal; 4/m 2/m 2/m; natroapophyllite is orthorhombic, 2/m 2/m 2/m. 
•	Crystal Habits include four sided prisms (with a square cross-section) truncated with either a steep four sided pyramid or a pinacoid termination or both. If the pyramids are missing, the crystals can look cubic. Rarely are the prisms missing, but if they are, crystals could appear octahedral because of the four sided pyramids. Cleavage is perfect in one direction (basal). 
•	Fracture is uneven. 
•	Hardness is 4.5 - 5. 
•	Specific Gravity is approximately 2.3 - 2.4 (lighter than most translucent minerals). 
•	Streak is white. 
•	Other Characteristics: Prism faces are striated lengthwise, some specimens are fluorescent and crystals will flake when heated. 
•	Associated Minerals are prehnite, quartz, heulandite, stilbite, natrolite, analcime, datolite, babingtonite, cavansite, calcite, idocrase, wollastonite, kinoite, gyrolite and many other zeolites. 

THE MINERAL CAVANSITE

•	Chemistry: Ca(VO)Si4O10(H2O)4, Hydrated Calcium Vanadium Silicate. 
•	Class: Silicates 
•	Subclass: Phyllosilicates 
•	Uses: mineral specimens. 
•	Specimens 
•	Cavansite is a beautiful and rare mineral. It was only discovered in the last 30 years and is found in only a few locallities. By far the best crystals come from the famous zeolite quarries in Poona, India. Crystal aggregates consist of spherical rosettes with jutting pointed crystals. The deep blue color of even the smallest cavansite crystals is truly amazing. A beautiful blue cavansite rosette perched on top of the muted colors of the typical zeolites makes a dramatic crystal association. Cavansite's rarity and beauty explain its recent popularity. 

PHYSICAL CHARACTERISTICS:
•	Color is greenish-blue to ocean blue.
•	Luster vitreous to pearly.
•	Transparency transparent to translucent.
•	Crystal System: Orthorhombic
•	Crystal Habits radiating acicular crystals forming spherical crystal clusters.
•	Cleavage perfect in one direction.
•	Fracture conchoidal.
•	Hardness 3 - 4
•	Specific Gravity is approximately 2.33
•	Streak is blue.
•	Associated Minerals include zeolites such as stilbite and heulandite as well as calcite, apophyllite , babingtoniteand quartz.
•	Other Characteristics: larger crystals show an unusual internal reflection.
•	Notable Occurrence is Poona, India and Columbia Co. and Malheur Co., Oregon, USA.
•	Best Field Indicators are color, associations, locality and crystal habit.

Tectosilicate (frameworks)

•	This subclass is often called the "Framework Silicates" because its structure is composed of interconnected tetrahedrons going outward in all directions forming an intricate framework analogous to the framework of a large building. In this subclass all the oxygens are shared with other tetrahedrons giving a silicon to oxygen ratio of 1:2. In the near pure state of only silicon and oxygen the mineral is quartz (SiO2). But the tectosilicates are not that simple. It turns out that the aluminum ion can easily substitute for the silicon ion in the tetrahedrons up to 50%. In other subclasses this substitution occurs to a more limited extent but in the tectosilicates it is a major basis of the varying structures. While the tetrahedron is nearly the same with an aluminum at its center, the charge is now a negative five (-5) instead of the normal negative four (-4). Since the charge in a crystal must be balanced, additional cations are needed in the structure and this is the main reason for the great variations within this subclass. Below are the more common members of the tectosilicate subclass. See the tectosilicates' page for a more complete list.

THE FELDSPAR GROUP

•	The feldspar group is a fairly large group with nearly 20 members recognized, but only nine are well known and common. Those few, however, make up the greatest percentage of minerals found in the Earth's crust. The following are some of the more common feldspar minerals: 

The plagioclase feldspars:
•	Albite, (Sodium aluminum silicate) 
•	Oligoclase, (Sodium calcium aluminum silicate) 
•	Andesine, (Sodium calcium aluminum silicate) 
•	Labradorite, (Calcium sodium aluminum silicate) 
•	Bytownite, (Calcium sodium aluminum silicate) 
•	Anorthite, (Calcium aluminum silicate) 

The K-feldspars or alkali felspars:
•	Microcline, (Potassium aluminum silicate) 
•	Sanidine, (Potassium sodium aluminum silicate) 
•	Orthoclase, (Potassium aluminum silicate) 

•	The feldspars are a group of minerals that have similar characteristics due to a similar structure. All feldspars have low symmetry, being only monoclinic, 2/m, to triclinic, bar 1. They tend to twin easily and one crystal can even be multiply twinned on the same plane, producing parallel layers of twinned crystals. They are slightly hard at around 6, and have an average density at 2.55 to 2.76. They have a rather dull to rarely vitreous luster. Crystals tend to be blocky. Some feldspars may be triboluminescent. They have two directions of cleavage at nearly right angles. Feldspars also tend to crystallize in igneous enviroments, but are also present in many metamorphic rocks. 

•	The general formula, for the common feldspars, is XAl(1-2) Si(3-2) O8 . The X in the formula can be sodium, Na and/or potassium, K and/or calcium, Ca. When the cation in the X position has a positive one (+1) charge such as with sodium or potassium, then the formula contains one aluminum and three silicons ions. If the formula contains the positive two (+2) cation calcium, then the formula will contain two aluminums and only two silicon ions. This substitution keeps the formula balanced, because aluminum has a charge of positive three (+3) and silicon has a charge of positive four (+4). Basically, the more calcium in the crystal, the more aluminum that will be needed to balance the charge. 

•	The silicons and aluminums occupy the centers of interlinked tetrahedrons of SiO4 and AlO4. These tetrahedrons connect at each corner to other tetrahedrons forming an intricate, three dimensional, negatively charged framework. The cations that represent the X in the formula sit within the voids in this structure. 

THE FELDSPATHOID GROUP

•	The feldspathoids are not a typical mineral group like other mineral groups that are related by structure and chemistry. Their relation to each other is due primarily to their relationship to the feldspar group of minerals. Feldspathoids are low silica igneous minerals that would have formed feldspars if only more silica (SiO2) were present in the original magma. The aluminum to silicon ratio is nearly 1:1 in most of the feldspathoids but is closer to 1:3 in most of the feldspars. As would be predicted, feldspathoids are not found in igneous rocks that contain primary quartz. 

•	Similar to the zeolites, the feldspathoids have large openings in the crystal structure. These openings are mostly separated from each other and do not lend themselves to the movement of ions and molecules that is allowed in the zeolite's structure. The openings do allow the structure to contain large ions like chlorine, carbonate and sulfate. The lower density common to the minerals of this group is evidence of the openess of their structure. 

•	The sodalite group is a group within a group. It is composed of minerals with a similar isometric structure and related chemistry. It is named for its most common member, sodalite. 

The following are some of the more common feldspathoid minerals: 
•	Cancrinite (Sodium Calcium Aluminum Silicate Carbonate) 
•	Leucite (Potasium Aluminum Silicate) 
•	Nepheline (Sodium Potasium Aluminum Silicate) 
•	Sodalite Group: 
•	Hauyne (Sodium Calcium Aluminum Silicate Sulfate) 
•	Lazurite (Sodium Calcium Aluminum Silicate (Sulfate Sulfide Chloride)) 
•	Nosean (Sodium Aluminum Silicate Sulfate) 
•	Sodalite (Sodium Aluminum Silicate Chloride)