February 27 FIRST LONG EXAM (Covering weeks 1 to 6)

PHYSICAL PROPERTIES OF MINERALS

• HABIT - the characteristic manner in which a mineral grows. We've already covered this in week 3.
• CLEAVAGE - the property of a mineral to break along planar, crystallographically controlled surfaces. Described by:
• number of cleavages - one, two, or more cleavages
• direction or orientation of cleavage - denoted by the form or plane that the cleavage is parallel to. (example: "parallel to {010}", or "octahedral {111} cleavage")
• quality of the cleavage - denoted by terms such as "poor", "good" or "perfect" or "excellent"
• Examples:
• plagioclase has two cleavages, {001} which is perfect and {010} which is good. (note: Because plagiocalse is triclinic, these two planes are not at 90 degrees; they are slightly inclined. Hence the name "plagiocalse" -- plagio ("inclined") and klasis ("breaking").
• orthoclase also has {001} and {010} cleavage. (But since orthoclase is monoclinic, these two planes are perpendicular, hence ortho ("at right angles"), klasis ("breaking")
• Halite has {100} perfect cleavage. Since halite is in the isometric system, we can infer that the {100} cleavage implies 3 cleavage directions all at right angles to each other ("cubic cleavage")
• Clinopyroxene (CPX) has good {110} cleavage. Since CPX is monoclinic with 2/m symmetry on can infer that the {110} cleavage implies 2 cleavage directions. Also, since the a and b unit cell dimensions are similar, the angle between the two {110} cleavages is near 90 degrees.
• Note 2: Cleavage refers to the manner in which a mineral might break, so just because a particular sample of a mineral doesn't obviously exhibit a cleavage surface(s), it doesn't mean the mineral doesn't have cleavage. (Ideally, one would want break the mineral with a hammer, say, to see how it breaks and determine if it has cleavage.)
• Note 2: Smaller grains of a mineral tend to display their cleavage less then larger grains. Hence, under the microscope one might not see any cleavage lines in small grains of CPX, say, but one might see several cleavage lines in larger grains.
• PARTING - The fracturing of minerals along planes of structural weakness that are not cleavage surfaces. 
• The most common causes of these planes of weakness are:
• twinning composition planes
• exsolution lamellae
• Examples:
• magnetite has no cleavage but shows octahedral parting because of {111} twinning.
• Orthopyroxene (OPX) may exhibit parting on {100} because of exsolution of CPX parallel to that plane
• FRACTURE - the manner in which a mineral breaks other than cleavage and parting.
• chonchoidal - smooth curved fractur surfaces resembling a shell; fracture of glass, quartz, olivine, and pyrite (often)
• fibrous/splintery
• hackly - having jagged fractures with sharp edges; ex.: native gold, silver, and copper
• uneven or irregular
• HARDNESS (H) - the resistance to scratching.
• Mohs scale of hardness
• Absolute hardness - diamond indentation test
• Note: When doing a scratch test,
• Make sure you are scratching a single crystal - many beginners scratch an aggregate of fine crystals plucking out individual grains thinking they have scratched the mineral.
• Make sure you are scratching a fresh surface. Weathering and alteration may produce a softer surficial layer on the mineral
• Make sure your "scratch" is a scratch. You may just be leaving a mark on the mineral (from your knife, say) and not really scratching it.
• TENACITY - the manner in which a mineral deforms under stress (at atmospheric P and T !).
• brittle - minerals that powder or fracture easily; many minerals with cleavage, parting and fracture are brittle.
• malleable, sectile, ductile - characteristic od metallic bonded minerals
• flexible - minerals that have foliated habits and can be bent, but don't resume their original shape (ex. chlorite, talc)
• elastic - same as above but resumes original shape (ex. micas)
• (Note: Of course the strain rate affects the tenacity of a mineral--minerals that are brittle at high strain rates at atmospheric P and T may deform in a ductile manner at very low strain rates.)
• SPECIFIC GRAVITY (G) (relative density) - ratio between the weight of a substance and the weight of an equal volume of water (at 4 C).
• Numerically equivalent to density in gm/cc (because water has density 1 gm/cc)
• How determined? G = Wa / (Wa - Ww), where Wa = weight of sample in air and Ww is weight of sample in water. (Wa - Ww is simply the weight of the equal volume of water.)
• G is dimensionless
• Example: quartz has a G of 2.65 (Its density is 2.65 gm/cc; it weighs 2.65 times the weight of an equal volume of water.)
• G depends on:
• The kinds of elements in the mineral (i.e. their atomic weight). Example: all the minerals in the table below are isomorphs, the only difference being the atomic mass of the large cation. Note the increasing G with increasing mass of the cation:

Mineral	Composition	A.M. of unique cation	G
aragonite	CaCO3	40.08 amu	2.94
strontianite	SrCO3	87.62 amu	3.78
witherite	BaCO3	137.34 amu	4.31
cerussite	PbCO3	207.19 amu	6.58

• The manner of packing of the ions. Take the following polymorphs as an example. Aragonite has a denser packing than calcite: 

Mineral	Composition	G
Calcite	CaCO3	2.71
Aragonite	CaCO3	2.94

• It is good to develop a "feel" for densities by remembering the G of some common minerals. (Note: gold, along with the much rarer mineral iridosmine (IrOs), G ~20,  has the highest density of all minerals.)

Mineral	Approx. G
halite	2.2
quartz, feldspars	2.7
hornblende	3.2
pyrite	5.0
gold	15-20 !

• COLOR - the response of the eye to the visible part of the electromagnetic spectrum. 
• Minerals can be classed as:
• allochromatic - due to a foreign colorant, i.e., a different mineral or some other phase as inclusions in the mineral (e.g., needles of rutile or tourmaline in quartz causing bluish color; fluid inclusions in quartz causing white color; small flakes of hematite in quartz casuing pinkish color).
• idiochromatic - "self colored"; color is caused by interaction of light with constituent ions, electrons of the mineral.
• Light can interact with a mineral by reflection, transmission (with possible refraction), scattering, or absorption. The most common cause of color in minerals is the absorption of certain wavelengths of light causing the reflected, transmitted or scattered light to be colored.
• Causes of absorption (of certain wavelenths of light) - to be continued ...