C. Special Mineral
Properties Related to Light, Heat, and Electricity
1. Play of Colors
(Iridescence)
-results from diffracted and reflected light from closely spaced fractures, cleavage
planes, exsolution, or minute inclusions in parallel orientation--
in opal--reflected light from submicroscopic layers of spherical SiO2
particles
in labradorite--reflected light from exsolution laminae
2. Chatoyancy
-silky or wavy appearance in minerals caused by closely packed parallel fibers
in satin spar--gypsum
in cat's eye--chrysoberyl
in tiger's eye--quartz
3. Asterism
-a
star like figure appearing on the surface of a mineral caused by reflection or scattering of
light off needle inclusions of minerals which parallel crystallographic axes
rubies and sapphires--corundum
4. Luminescence
-a special glowing of the mineral caused by the presence of impurities (activators) in a mineral
a.
Fluorescence
-emission of light wavelength caused by electron excitation in transition metals
during bombardment of a mineral with ultraviolet or x-rays-- examples are:
some specimens of fluorite, scheelite, calcite
b.
Phosphorescence
-continuous emission of light wavelength after the excitation source is removed-
similar mineral specimens as given in fluorescence above
5. Thermoluminescence
-emissions of light wavelengths when mineral is subjected to heat below the "red
hot"
temperature--examples are:
some specimens of calcite and apatite
6. Piezoelectricity
-a flow of electrons in mineral crystals by simply exerting a compressive pressure at both
ends of the "c" axis of crystals which have no center of symmetry--
examples are:
quartz and tourmaline
7.
Pyroelectricity
-same as piezoelectricity except stimulated by heat
II. Hardness
A. Moh's Hardness Scale
-a listing of minerals
with increasing relative hardness 1-10-
1. talc
6. feldspar
2. gypsum
7. quartz
3. calcite
8. topaz
4. fluorite
9. corundum
5. apatite
10.diamond
-the following can be
assigned a hardness in this scale-
glass = 5.5;
knife = 5.5; steel file = 6-7;
fingernail = 2.5; penny = 3
-the hardness of a mineral can
differ slightly in the direction of scratching--kyanite between
a hardness of 5 to 7, and calcite between 2
and 3
III. Tenacity
--the cohesiveness of a mineral, or the
resistance a mineral offers to breaking, crushing,
bending or tearing--
1. brittle--if a mineral powders easily
2. malleable--if a mineral can be hammered into
sheets
3. sectile--if a mineral can be cut into thin
shavings with a knife
4. ductile--if a mineral can be drawn into wire
5. flexible--if a mineral is bent but does not resume
the original shape
6. elastic--if a mineral bends and resumes the
original shape
IV. Streak
--is the color of the powder from the
mineral on a porcelain plate (streak plate)--
--since most minerals are softer than porcelain
(8 hardness on Moh's Scale), their powder
will remain after scratching it on the
streak plate
V. Luster
--is the general appearance of the
surface of a mineral in reflected light--minerals can have
a metallic luster if
they are opaque to light, give a prominent dark or black streak, and are
usually dense like galena,
pyrite, and bornite or can have a non metallic luster if they are
translucent or transparent to light and
have a colorless or white streak
--on occasion it is difficult to determine if a
mineral is one or the other and may have a
sub metallic luster
--some specific types of non
metallic lusters are:
1. Vitreous-resembles
glass--quartz crystals
2. Resinous-resembles
a resin--sulfur and sphalerite
3. Pearly-pearl-like
and present on mineral surfaces paralleling cleavage planes--talc
4. Greasy-appears
to be covered by oil or grease--massive quartz
5. Silky-silk
or satin-like--satin spar gypsum
6. Adamantine-brilliant
looking minerals which have a high index of refraction--diamond
VI. Cleavage, Parting, Fracture, Specific Gravity and Crystal Habit
A. Cleavage
-the
ability of a mineral to break or come apart in a consistent way--breakage is along
atomic
planes-cleavage is consistent with crystal symmetry and may be one to multi-
directional
from one mineral to another-
--micas = one direction of cleavage
--feldspars, pyroxenes and amphiboles = two directions of cleavage
--calcite and dolomite = three directions of cleavage (rhombohedral)
-cleavage may be of various qualities as perfect, good, fair or poor but the same
cleavage quality and quantity will be present in all specimens of the same mineral
-the
presence of cleavage in a mineral is often observed by reflecting light off a fresh
surface and observing a stair like arrangement of thin parallel layers
B. Parting
-breakage of minerals
along planes of structural weakness such as twinning planes
-unlike cleavage,
parting is not shown by all specimens of the same mineral but
only those
which are twinned or formed under special pressure conditions
C. Fracture
-the
inability
of a mineral to break in a consistent way and therefore not along cleavage
planes
-a few specific types
of fracture are:
1. Concoidal- a smooth, curved breakage--quartz
2. Fibrous or Splintery
3. Hackly-jagged with sharp edges
4. Irregular-rough surfaces
D. Specific Gravity
-a
number that expresses a ratio between a substance and the weight of an equal volume
of water at
4 degrees C--the number is the same for that of the density without units
-specific gravity
(S.G.) depends on (1) the kind of atoms comprising a mineral (atomic
weight) and
(2) the packing of the atoms (close packing or loosely packed)
-S.G. can be
measured with a Jolly Balance which determines the weight of a mineral in
air and the
loss of weight in water--the S.G. number is then obtained by dividing the
weight in
air by the loss of weight in water
-S.G.
can also be calculated--
E. Crystal Habits and Aggregates
-the appearance of a
single crystal or an aggregate of crystals of a mineral can aid in the
identification of the
mineral
1. isolated
individual mineral crystals
a. bladed-elongated flattened crystals looking like a knife
blade
b. acicular-thin needlelike crystals
c. capillary-hair like or thinner
2. groups
of distinct crystals
a. dendritic-resembling small veins on a tree leaf
b. radiated-crystals appearing in a radial pattern
c. drusy-a surface containing very small crystals
3.
groups of distinct crystals occurring in parallel or spherical form
a. columnar-column like crystals
b. bladed-many flat bladed crystals
c. fibrous-parallel fibers
d. colloform-includes botryoidal ( resembling a bunch of
grapes), reniform (kidney-
shaped masses, and mammillary (mammae masses)
4.
aggregate of scales or lamellae
a. foliated (micaceous)-easily splits into thin leaves or
sheets
b. plumose-scaley-featherlike structure
5.
granular aggregate
comprised of an aggregate of equant grains
6.
other types
a. pisolitic and oolitic-rounded masses of pea size grains
(pisolitic) or very small
grains (oolitic)
b. stalactitic-resembling stalactites
c. massive-massive with no form or distinguishing features
d. geode-rock cavity filling with mineral crystal
e. banded-a mineral showing narrow bands of different colors
or textures
|