THE CASE OF THE SCARLETT STREAK:
MINERAL IDENTIFICATION
A mineral is a naturally occurring, inorganic, solid, homogeneous, crystalline material with a
unique chemical element or compound with a set chemical formula; usually obtained usually
from the ground. Minerals have distinctive physical and chemical properties that allow for their
identification.
These are the physical properties most useful for mineral identification (bold indicates most
important for visual identification):
 Habit (Crystal forms and shapes)
 Hardness
 Cleavage
 Streak
 Color
 Luster
 Transparency
 Twinning
 Fracture
 Specific Gravity
 Associated Minerals
Other Important Properties and Characteristics:
 Fluorescence
 Magnetism
 Odor
 Feel
 Taste
 Solubility
 Reaction to acids
 Radioactive minerals
 Meteoritic minerals
Those physical properties of minerals which can easily be determined by inspection or simple
tests and are thus useful in mineral identification include:
1
HABIT (CRYSTAL SHAPE)
Minerals can be grouped into six crystal systems based on their internal symmetry. These
include triclinic, monoclinic, orthorhombic, tetragonal, hexagonal and isometric in order of
increasing symmetry. When conditions for crystal growth are favorable, the ordered internal
arrangement of minerals is expressed outwardly by smooth faces and regular geometric forms.
The external appearance or shape of a mineral crystal is known as its habit. The appearance of
crystals either singly or in groups (mineral aggregates), is extremely useful in the identification
of minerals.
The environment in which a crystal grows determines its habit. A mineral can exhibit more
than one habit depending on the conditions under which it formed. Under ideal conditions
crystals would be perfect geometric forms. Most crystals are malformed to some degree, and the
degree of crystal perfection also depends on the conditions of formation.
Commonly recognized crystal habits include prismatic, cubic, octahedral (8-sided), and
dodecahedral (12-sided). Mineral crystals (or aggregates) can also be described as being acicular
(needlelike), bladed (flattened, elongate), radiating, fibrous, tabular (flat, plate like), dendritic
(branching), botryoidal (globular, grapelike), reticulated (lattice like), etc.
HARDNESS
One of the most useful properties for identifying minerals is hardness. Hardness is the
resistance of a mineral to being scratched. A German mineralogist by the name of Friedrich
Mohs, worked out a scale of hardness for minerals. Using ten common minerals, he arranged
them in order of increasing relative hardness, with number one assigned to the softest mineral
and ten assigned to the hardest. A mineral assigned a given hardness on Mohs scale can scratch
all minerals lower on the scale.
Mohs Scale
Mineral
Talc
Gypsum
Calcite
Fluorite
Apatite
Feldspar
Quartz
Topaz
Corundum
Diamond
Hardness
1
2
3
4
5
6
7
8
9
10
The approximate hardness of an unknown mineral specimen can be determined by testing it
using the minerals in Mohs scale. To do this, you scratch the unknown mineral against the
surface of each of the other minerals. For example, if the unknown mineral scratches calcite
(H=3), but will not scratch fluorite (H=4), we can say its hardness is somewhere between 3 and
4. If neither the known nor the unknown mineral scratches each other, they have the same
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hardness. You may not always have the minerals in Mohs scale so we have listed simple
additional tests for determining approximate hardness.
Hardness
1
2
3
4
5
6
7
Field Test
Easily scratched by fingernail
Scratched by fingernail (H=2.5)
Scratched by a copper penny (H=3)
Scratched easily by a knife (H just over 5)
Difficult to scratch with a knife; barely scratches glass (H=5.5)
Scratched by a steel file (H=6.5); easily scratches glass
Scratches a steel file and glass
When testing minerals, a soft mineral may make a mark on a harder mineral that looks like a
scratch but which is actually the streak of the softer mineral. Always test to see if you can rub it
off. You can’t rub off a scratch. It is wise to test both ways. Make sure you test a fresh surface,
since weathered surfaces may have a coating of softer alteration minerals.
CLEAVAGE
Cleavage (or lack of cleavage) is another useful property used to determine the identity of a
mineral. Cleavage is a property of some minerals that describes how they split or break. If a
mineral splits along definite planes leaving flat surfaces, it has cleavage. Depending on how well
developed it is, cleavage can be described as perfect, good, fair, etc. Examples of minerals with
cleavage are halite, which cleaves in three directions, forming cubes and mica, which cleaves in
only one direction, forming thin elastic sheets. Many minerals do not split evenly, but instead
fracture, or break unevenly. These minerals do not have cleavage. Quartz is an example of a
mineral that lacks cleavage, instead fracturing like broken glass (conchoidal fracture).
STREAK
Streak is another physical property used to help identify minerals. Streak is the color of the
mineral powder left by a specimen after it is rubbed against a hard porcelain plate or streak plate
(H  7). Although the color of a mineral may vary, the streak of a mineral is always the same.
Additionally, The color of the streak will not necessarily be the same as the color of the mineral.
Minerals that are light colored may exhibit a white or colorless streak and those with hardness
greater than 7 will not leave a streak. For such minerals, other physical properties need to be
used to help identify them.
COLOR
In some minerals, color is a fundamental property and is constant and characteristic. For
such minerals, color serves as an important means of identification. For example, the minerals,
malachite and azurite are always green and blue, respectively. Most metallic minerals also show
constant color. The majority of minerals, however, display a variety of colors, and it is not
possible to rely on color alone to identify them.
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Many minerals can have traces of different elements as impurities that produce color. For
example, the colorless mineral, corundum takes on a reddish color when traces of chromium are
present. The reddish gem variety of corundum is called ruby. Sapphire is the blue gem variety of
corundum produced by traces of titanium and iron.
When identifying a mineral by color, one must also consider the possibility that surface
tarnish may have affected the color on different surfaces.
LUSTER
Minerals reflect, bend, or absorb light to different degrees. Luster is the physical property of
a mineral that refers to its appearance in reflected light. Certain minerals reflect light the way a
highly polished metal does. Such minerals (i.e. silver, gold, copper, and galena) have “metallic”
luster.
Minerals that are not highly reflective but which instead absorb or transmit light are
described as having “nonmetallic” luster. Some terms used to describe common nonmetallic
lusters include resinous, silky, pearly, vitreous (glassy), waxy, dull, adamantine (brilliant) or
satiny.
SPECIAL PROPERTIES
Certain minerals have additional properties that will help you identify them.
1. Magnetism: Magnetite (an iron ore) and some samples of pyrrhotite (pir-oh-tyt) are the
two common minerals attracted to a magnet. Lodestone is a variety of magnetite that acts as a
natural magnet. The magnetic properties of lodestone have been known since ancient times.
2. Fluorescence: Some minerals glow when exposed to ultraviolet (black) light. This
phenomenon is usually observed in minerals that contain foreign ions called activators. These
activators absorb ultraviolet light and give off visible light rays. Some minerals fluoresce under
short wave UV, some under long wave UV and others under either wavelength. The property is
unpredictable, the color of emitted light may vary and not all specimens exhibit the property,
even those from the same locality. Minerals that continue to glow for a time after the source of
ultraviolet light is turned off are said to be phosphorescent. Some minerals emit visible light
when heated (thermoluminescence), or upon being crushed, scratched or rubbed
(triboluminescence).
3. Radioactivity: Minerals that contain radium and uranium are radioactive and usually also
fluoresce. The radioactivity can be detected with a geiger counter.
4. Fizzes with application of 10% HCl acid (calcite).
4
#1
Graphite
C
(element)
Crystal System:
Hexagonal
Appearance and
Physical Properties:
Graphite is most easily recognized by its black to dark silvery-grey color,
softness (H=1), and slippery feel. It has a silvery-black streak, and is quite
light. Crystalline graphite is rare; it is more commonly found as thin layers
or massive lenses. Greasy to submetallic luster. It has perfect cleavage in
one direction.
Environment:
Graphite is formed from amorphous or biological carbon under intense
heat and pressure; most deposits are found in metamorphic rocks formed
from sedimentary rocks. The world’s largest graphite deposits are in Sri
Lanka (Ceylon) where they are found as lenses within schist. The black
veining in white decorative marble is often graphite.
Uses:
Besides being used to make the lead in pencils (pencil leads are actually
graphite mixed with clay or other binders to make it harder), graphite is
also used in making iron and steel, and in controlling nuclear reactions
inside nuclear power plants.
#2
Galena
PbS
(Sulfide)
Crystal System:
Isometric
Appearance and
Physical Properties:
The most common crystal form of galena is the cube, which is sometimes
truncated by the octahedron. Also massive. Galena is easily recognized by
its blue-black to silver gray color, black streak, bright metallic luster,
softness (H=2.5), its perfect cubic cleavage and most notably by its high
specific gravity (very heavy).
Environment:
Common sulfide found in hydrothermal veins associated with other sulfide
minerals, carbonates and gangue minerals (quartz, barite, fluorite).
Frequently associated with silver minerals and may contain silver in its
structure. Also occurs as a low temperature replacement deposit or in
veins in limestones (Mississippi Valley type deposits), in contact
metamorphic deposits, pegmatites and disseminated in sedimentary rocks.
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Uses:
Main ore of lead. Used in the manufacture of batteries and metal products,
for solder and as a protective shield for radioactive materials. Galena is
also an important silver ore.
#3
Pyrite
FeS2
(Sulfide)
Crystal System:
Isometric
Appearance and
Physical Properties:
Frequently in crystals, the most common forms of which are cubes,
pyritohedrons, and octahedrons. Also occurs as compact granular
aggregates. Pseudomorphs after organic fossil remains common. Pyrite is
opaque with a characteristic pale brass yellow color and bright metallic
luster. It has a greenish or brownish-black streak and is hard (6–6.5) and
brittle with a conchoidal fracture.
Environment:
Most common and widespread sulfide mineral. Common in plutonic,
volcanic, sedimentary and metamorphic rocks. Frequently associated with
galena in hydrothermal veins.
Uses:
Chiefly used as a source of sulfur in the manufacture of sulfuric acid and
copperas (ferrous sulfate). Also used in inks, dyes, wood preservative and
disinfectants.
#4
Magnetite
Fe3O4
(Oxide)
Crystal System:
Isometric
Appearance and
Physical Properties:
Forms perfect octahedral crystals, but is more commonly found as
compact, fine to coarse granular masses, sometimes with a bluish
iridescence. Magnetite is black and opaque with a metallic luster and black
streak. Hard (6), with a high specific gravity, it is also strongly magnetic,
sometimes acting as a natural magnet (lodestone).
Environment:
Magnetite is a common accessory in igneous rocks. It may form large ore
bodies through magmatic segregation. Commonly associated with
crystalline metamorphic rocks. Extensive sedimentary deposits of banded
iron formation were formed in the Lake Superior Region during the
Precambrian. Large masses also found in detrital sedimentary rocks and
dune deposits.
6
Uses:
One of the richest and most important ores of iron.
#5
Hematite
Fe2O3
(Oxide)
Crystal system:
Hexagonal-R
Appearance and
Physical Properties:
Crystals are typically tabular with triangular markings on basal planes.
Usually massive, compact to earthy masses, sometimes with iridescent
surfaces. Often oolitic, botryoidal or concretionary in appearance. May
also appear micaceous and foliated as in the variety specularite.
Hardness (5.5–6.5). Luster is metallic in crystals, dull in earthy varieties.
Characteristic reddish-brown streak. Often colors both rocks and minerals
shades of red and reddish-brown.
Environment:
Hematite is formed primarily as a result of the weathering and oxidation of
iron-bearing minerals and rocks. It is widely distributed in all rock types;
occurring in metamorphic rocks, as an accessory in igneous rocks, and in
sedimentary rocks either as cement or in large economic deposits formed
when silica is leached, leaving behind Fe-rich ore. Much hematite is
formed under sedimentary conditions through digenesis of limonite,
retaining its concretionary and oolitic forms.
Uses:
The most abundant and important iron ore. Red ochre, the soft, earthy
variety of hematite, is used as a pigment and a polishing compound.
#6
Halite
NaCl
(Halide)
Crystal System:
Appearance and
Physical properties:
Cubic
Halite crystals are almost always perfect cubes, although they can also be
found as ‘hopper crystals’ shaped like inside-out step pyramids. Halite is
also found as massive layers or lenses. Halite has a waxy to greasy luster;
it is white in its pure form but can be easily colored pink, red, yellowish or
even dark blue by various kinds of inclusions. Halite is easy to recognize
by its salty taste. It has a hardness of 2.5 and perfect cleavage in three
directions.
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Environment:
Halite is formed by the evaporation of salty water. This can occur in
drying sea beds, lake beds, or even in tiny inclusions of fluid within other
rocks. The largest halite layers occur in sedimentary rocks related to
drying seas; these can be thousands of feet thick. Halite often occurs
around deposits of oil and native sulfur.
Uses:
Besides being used for table salt, halite is used as a de-icing treatment on
roads in areas with cold climates. It can be separated to produce native
sodium and chlorine; sodium is used in streetlights and many chemical
processes, and chlorine is used to disinfect water and to produce toxic
gases (and many other chemical processes).
#7
Fluorite
CaF2
(Halide)
Crystal System:
Isometric.
Appearance and
Physical Properties:
Fluorite usually occurs in cubes or modified cubes that often exhibit
penetration twins. Color is extremely variable; from colorless and
completely transparent when pure, to yellow, green, blue, pink, purple,
brown and even black. Transparent to translucent with vitreous luster.
Moderate hardness (4) with perfect octahedral cleavage. Some varieties
are fluorescent.
Environment:
Fluorite is a common and widely distributed mineral. It occurs chiefly in
hydrothermal veins as a main mineral or as a gangue mineral associated
with lead and silver ores. Common in dolomites and limestones and as a
minor accessory mineral in igneous rocks and pegmatites.
Uses:
Source of fluorine for the production of hydrofluoric acid, which is vital in
the pottery, optical and plastics industries. Also used as a flux in the metal
industry. Very clear crystals (optical calcite) are used as lenses and as
spectrographic prisms.
#8
Calcite
CaCO3
(Carbonate)
Crystal System:
Hexagonal-R
Appearance and
Physical properties:
Crystals extremely varied in form (habit). Important forms include
rhombohedral, scalenohedral, or prismatic crystals, often twinned. Usually
in crystals or in coarse to fine-grained aggregates. Also compact, earthy,
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stalactitic. Color varied. Often colorless to white; can be gray, blue, pink,
green, yellow, or brown to black if impure. Transparent to translucent with
vitreous to earthy luster and a white streak. Hardness (3). Perfect
rhombohedral cleavage. Some varieties are fluorescent in ultraviolet light.
Effervesces in dilute hydrochloric acid.
Environment:
One of the most common and widespread minerals. Major constituent of
sedimentary limestones and dolomites, and of marbles (metamorphosed
limestone). Also occurs in hydrothermal veins and as a cement in
calcareous marls and sandstones. Primarily formed by the deposition
and consolidation of calcareous shells and skeletons on the sea floor. Also
formed by chemical precipitation through the evaporation of solutions rich
in calcium bicarbonate, such as cave deposits and spring deposits
(travertine).
Uses:
Most important use is in Portland Cement and lime for mortar. Also used
as a flux for smelting and in the building industry as an aggregate and
ornamental stone. Clear, colorless crystals (iceland spar) are used to make
polarizing prisms.
#9
Azurite
Cu3 (CO3)2(OH)2
(Carbonate)
Crystal System:
Appearance and
Physical Properties:
Environment:
Monoclinic
Azurite is a very distinctly colored mineral; it is always blue, from pale
baby blue in intense midnight blue. Crystals of azurite are not uncommon;
they are often very dark blues with a vitreous luster. Azurite is also founds
as rounded masses that can have an earthy or dull luster; these are often
paler blue. Azurite can effervesce slowly in dilute hydrochloric acid;
sometimes a clean iron nail rolled in the acid will develop copper plating.
Azurite can also distinguished from chrysocolla, a similar blue mineral,
because when you put your tongue on chrysocolla, it sticks. Azurite,
which doesn’t have the same ‘spongy’ structure that chrysocolla does, will
not stick to your tongue. It has a hardness of 3.5-4 and perfect cleavage in
one direction.
Azurite usually forms when other copper minerals oxidize; in the
Southwest, it is very common in the upper portions of porphyry copper
and carbonate replacement deposits.
9
Uses:
Azurite is used as a minor ore of copper, and also as a decorative stone
and in jewelry. Powdered azurite is a brilliant blue pigment which has
been used in paints and fabric dyes for thousands of years.
#10
Gypsum
CaSO4·2H2O
(Sulfate)
Crystal System:
Monoclinic
Appearance and
Physical Properties:
Crystals typically diamond-shaped, tabular, with beveled edges.
Swallowtail twins common. Often clear to white; also gray, yellowish or
brown. Soft (H=2), scratches easily with fingernail. Three unequal
cleavages; one perfect yielding slightly flexible but inelastic plates, one
with fibrous fracture and one with conchoidal fracture. Transparent to
translucent, with vitreous luster typical. May also be compact, massive
with no visible cleavage (alabaster) or fibrous with a silky luster (satin
spar).
Environment:
Most common sulfate mineral. Widely distributed in sedimentary rocks.
Form as large evaporite bodies, as lenses or interlayered with other
sediments. Common gangue mineral in metallic veins. Also occurs in
volcanic regions from the interaction of sulfur vapors on limestone.
Uses:
Primarily for the manufacture of plaster of Paris that is used in wallboard,
molds and casts. Also as a flux in pottery and as a fertilizer. Some
varieties of alabaster used in interior decoration and sculpture.
#11
Quartz
SiO2
(Silicate)
Crystal System:
Hexagonal
Appearance and
Physical Properties:
Frequently occurs in well-formed crystals that are usually hexagonal and
prismatic, often with horizontal striations on the prism faces. Commonly
twinned. Crystals are either right- or left-handed, which is caused by the
spiral structure of linked silicon-oxygen tetrahedra. Quartz can also be
microcrystalline to cryptocrystalline, occurring in compact or
concretionary masses.
In addition to its crystal form, quartz is recognized by its hardness (7) and
conchoidal fracture. Colorless quartz is called rock crystal. Other varietal
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names based on color include amethyst (purple), citrine (yellow), smoky
(brown), milky (white, opaque), and rose (pink) quartz. Coarse crystalline
varieties are transparent to translucent with vitreous luster. Microcrystalline to cryptocrystalline varieties are fibrous (chalcedony, i.e. agate)
to granular (flint, chert, jasper), with dull to waxy luster.
Environment:
One of the commonest minerals of the earth’s crust (12 percent by
volume), quartz occurs in a variety of igneous and metamorphic rocks. It
is a major component of granite pegmatites. A common hydrothermal vein
mineral, it is the most frequent gangue mineral associated with metalbearing veins. Also stable in sedimentary environments where it either
precipitated from solution or is present as a detrital mineral (all alluvialmarine and desert sands) or as a cement in consolidated rock (sandstone).
Uses:
Used in large quantities in the manufacture of glass, paints, abrasives,
refractories and precision instruments. Attractive varieties are used as
semi-precious gems and for ornamental stones.
#12
K-Feldspar
KAlSi3O8
(Silicate)
Crystal System:
Triclinic (microcline); Monoclinic (orthoclase)
Appearance and
Physical Properties:
Occurs as short prismatic crystals, frequently twinned. Also in coarse
cleavable to granular masses and as irregular grains in rocks.
Hardness (6). Two prominent cleavage directions at right angles (90°
cleavage). Translucent to transparent with vitreous luster. Microcline is
commonly white to pale yellow, rarely green to blue green (amazonite).
Orthoclase can be colorless, white, or gray but is often a characteristic
fleshy pink color.
Environment:
The low temperature polymorph (microcline) is the common feldspar of
slowly cooled granites, syenites and pegmatites formed at low
temperatures and metamorphic gneisses. Orthoclase, the intermediate
temperature polymorph, is a major constituent of intermediate depth,
medium temperature granites, granodiorites, and syenites. K-feldspar is
often associated with quartz.
Uses:
Common rock forming mineral. Moonstone, a variety of K-feldspar which
shows an opalescent play of colors, is a semi-precious stone.
#13
11
Talc
Mg3(Si4O10)(OH)2
(Silicate)
Crystal System:
Appearance and
Physical Properties:
Orthorhombic
Talc is most easily identified by its soft, greasy feel and satiny luster. It is
white to pale yellow to pale green in color, and can be grey as well. Talc is
very soft (H=1) and fairly light. It can be found as crystals, but is most
common as earth masses or mixed with other minerals, most often quartz,
dolomite, and serpentine. It has perfect cleavage in one direction.
Environment:
Talc is most often formed from the metamorphosis or metasomatization
(modification by heat, pressure, and water) of dolomite. A rock formed
entirely of massive talc is sometimes called ‘steatite’.
Uses:
Talc is extremely resistant to heat, electricity, and acids, making it very
useful in making electrical switchboards and laboratory countertops. It is
also used as a filler material in paints, rubber, and insecticides. Talc is also
used in talcum powder and in various other cosmetics.
#14
Muscovite
KAl2 (AlSi3O10)(OH)2
(Silicate)
Crystal System:
Monoclinic
Appearance and
Physical Properties:
Distinct crystals rare, usually tabular with a diamond-shaped or
hexagonal outline, often with horizontal striations on the prism faces.
More typically foliated in small to large sheets. Also cryptocrystalline and
compact massive. Soft (2–2.5) and light in color, with highly perfect basal
cleavage allowing it to be split into very thin, flexible, and elastic sheets.
Environment:
One of the most common minerals in rocks, especially plutonic igneous
rocks rich in silica and aluminum (granites and granite pegmatites) and
low to high grade metamorphic rocks (mica schists and gneisses).
Uses:
Valuable as an insulating material in the manufacture of electrical
products due to its high electrical resistance and heat resisting properties.
Provides a shiny luster to wallpapers, paper and makeup products. Also
used in porcelain products, as a dry lubricant, and as a filler. Historically
used as a substitute for window glass.
#15
12
Augite
(Ca, Na) (Mg, Fe, Al) (Si, Al)2 O6
(Silicate)
Crystal System:
Appearance and
Physical Properties:
Orthorhombic
Augite is most commonly seen as a black, green-black, or brown-black,
dull, blocky crystals, although it can also be formed as compact or
granular masses, or even columnar, lamellar or fibrous forms. Augite is
fairly hard (H=5-6.5), but it is one of the first minerals to be destroyed in
the weathering environment. The name Augite really refers to a series of
minerals ranging in composition from Hedenbergite (CaFeSi2O6) to
diopside (CaMgSi2O6). Augite is also a member of the larger pyroxene
group of minerals.
Environment:
Augite is common in mafic and intermediate igneous rocks, and can
present although far less abundant in felsic igneous rocks as well. Augite
can also be formed by metamorphic or metasomatic processes, especially
in the high temperature portions of skarn systems.
Uses:
Augite is a common rock forming mineral. Exception specimens show a
brilliant vitreous luster and can be valuable to collectors.
#16
Hornblende
(Ca,Na) 2-3 (Mg,Fe,Al)5 (Al,Si)8 O22 (OH)2
(Silicate)
Crystal System:
Monoclinic
Appearance and
Physical Properties:
Usually in short to elongate prismatic crystals with a diamond-shaped
cross section. Hornblende is commonly dark green to black and
translucent with a vitreous luster. Fibrous varieties often have a silky
luster. Streak is colorless. Hardness (5-6). Perfect prismatic cleavage. The
two prominent cleavage directions intersect at angles of 56° and 124° to
form a diamond-shaped cross section. The fracture of hornblende is
uneven and splintery and can be very brittle.
Environment:
Hornblende is an important and widely distributed rock-forming mineral
of igneous and metamorphic rocks. It is an essential mineral in granite,
granodorite, diorite, and monzonite and their volcanic equivalents. A rare
plutonic rock composed entirely of hornblende is called hornblendite.
Hornblende is also a primary constituent of metamorphic amphibolites.
Uses:
Hornblende is a basic rock-forming mineral.
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#17
Biotite
K (Mg,Fe)3 (AlSi3O10) (OH)2
(Silicate)
Crystal System:
Monoclinic.
Appearance and
Physical Properties:
Usually occurs in irregular foliated masses; rarely in tabular or short
prismatic crystals with prominent basal cleavage and a pseudo-hexagonal
outline in cross section. It is dark brown, green to black with a splendent
luster. Biotite is soft, H (2.5–3). It has perfect basal cleavage (micaceous)
that appears as many thin elastic sheets that peel very easily.
Environment:
Biotite forms under a wide variety of temperature and pressure conditions
and is an important and common mineral of many igneous rocks, from
granites and pegmatites, to gabbros and peridotitites, as well as their
volcanic equivalents. It is also an important constituent of many
metamorphic rocks.
Uses:
One of the major rock-forming minerals.
#18
Topaz
Al2SiO4 (F,OH)2
(Silicate)
Crystal System:
Orthorhombic
Appearance and
Physical Properties:
Commonly in prismatic crystals that can be enormous. Often with
striations on vertical prism faces. Color is highly variable; ranging from
colorless, to yellow, blue, green, violet, or reddish-yellow. Transparent to
translucent with vitreous luster. In addition to its crystals, recognized
chiefly by its superior hardness (8), its high specific gravity, and perfect
basal cleavage.
Environment:
Characteristic of pegmatite deposits, where it forms from residual
fluorine-bearing vapors during the last stages of the crystallization of
siliceous magmas. Also occurs in cavities (vugs) in rhyolites (Topaz
Mountain, Utah). Because of its chemical resistance, hardness and high
specific gravity, it occurs as placer deposits in streams, occurring as round
translucent pebbles.
Uses:
Clear or finely colored varieties of topaz are faceted for use as gemstones.
The deep-golden variety is the most popular with blue or green varieties
also popular. Topaz is the Utah State gemstone.
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#19
Tourmaline
(Na,Ca) (Li,Mg,Al) (Al,Fe,Mn)6 (BO3)3 (Si6O18) (OH)4
(Silicate)
Crystal System:
Hexagonal
Appearance and
Physical Properties:
Exhibits prismatic crystals that are often elongated with vertical striations
and which appear triangular in cross-section. Color is commonly black,
but may be brown, green, yellow, red-pink, blue, white or clear.
Transparent to translucent with vitreous (glassy) luster. Hardness (7–7.5).
Conchoidal fracture.
Environment:
Most common occurrence is in granitic pegmatites in which the crystals
can sometimes be enormous. Common accessory in igneous and
metamorphic rocks.
Uses:
Used in the manufacture of high-pressure gauges, making use of the
mineral’s strong piezoelectric property. Gem varieties are sold as
semiprecious stones.
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MINERAL KIT
Teacher Answer Kit
1
2
3
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6
7
8
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graphite
Galena
Pyrite
Magnetite
Hematite
Halite
Fluorite
Calcite
Azurite
Gypsum
Quartz
potassium feldspar
Talc
Muscovite mica
Augite
Hornblende
Biotite mica
topaz
Tourmaline (black)
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EXCERSISE
Use the various mineral identification tests to identify a set of minerals, using a flowchart.
PROCEDURE
Follow the flowchart.
Step 1: Is the Luster Metallic or Nonmetallic? Use your hand lens to be sure.
Step 2: What is the hardness?
Will the mineral be scratched by a fingernail (Hardness less than 2½?)
Will the penny scratch the mineral (harness less than 3)?
Will the mineral scratch glass (hardness less than 4.5)?
Will the mineral scratch the streak plate (hardness of greater than 7.5)?
Step 3: What is the streak?
Step 4: What is the cleavage?
Step 5: What is the color?
Step 6: Other tests—fizzes with acid, magnetic, feel, taste, crystal shape.
EQUIPMENT
1. A copper penny (or small –½ inch – piece of copper or short piece of heavy copper wire).
2. A "streak plate" (unglazed porcelain tile – a 2 inch square is plenty).
3. A small magnet (a refrigerator magnet is fine, but should be a fairly strong one).
4. A glass plate.
5. A 10x hand lens/jeweler’s loupe.
6. A steel nail.
7. 10% HCl (for instructor use only).
MORE INFORMATION
http://www.ironorchid.com/minerals/
http://geology.csupomona.edu/alert/mineral/minerals.htm
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