Chapter 2.3
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Properties by which we can identify minerals
are:
Color
 Streak
 Luster
 Crystal Form
 Hardness
 Fracture
 Cleavage
 Density
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Some minerals have characteristic colors that
make them easy to identify
Pyrite or “fool’s gold” is always golden in color
 Pure sulfur is always yellow
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However, color is not that reliable because
slight changes in composition can drastically
change a mineral’s color.
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Some minerals come in many different colors.
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Streak is the color of a mineral’s powder.
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Sometimes a mineral’s streak is different than the
color of the mineral itself
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We test for streak by scratching the mineral against
an unglazed piece of porcelain called a streak plate
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When we have two samples with the same color,
testing their streak can help distinguish them
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The luster of a mineral is the way in which its
surface reflects light
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How shiny is this mineral?
Types of luster include
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Metallic
Adamantine (brilliant)
Greasy
Earthy
Waxy
Silky
Pearly
Vitreous (glassy)
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Crystal form is the visible expression of a
mineral’s internal arrangement of atoms.
There are 6 systems:
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Cubic
Tetragonal
Monoclinic
Triclinic
Hexagonal
Orthorhombic
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To develop a crystal with well formed faces,
the mineral must have time to develop slowly
and without space restrictions
This normally doesn’t happen because
minerals are crowded and compete for space
Crowding results in an intergrown mass of tiny
crystals, none of which shows its form
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Hardness is a measure of a mineral’s resistance
to being scratched
The Mohs Hardness Scale consists of 10
minerals arranged from the softest (1) to the
hardest (10)
Harder minerals will scratch softer ones, but
not the other way around
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To test for hardness we attempt to scratch an
unknown sample with minerals or other
objects whose hardness is known
Sometimes we use everyday objects with
known hardnesses:
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Finegernail- 2.5
Copper penny- 3.5
Knife- 5.1
Nail/ Glass- 5.5
Streak Plate- 6.5
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Cleavage is the tendency for a mineral to
cleave, or break, along flat, even surfaces.
The way a mineral breaks is determined by the
organization of its crystals.
Minerals will break where the bonds that hold
crystals together are weak.
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Minerals that do not show cleavage when
broken are said to fracture
Fracture is the uneven breakage of a mineral
Quartz, for example, has strong bonds in all
directions so instead of cleaving it fractures.
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Quartz’s fracture is called conchodial
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Density is a property of all matter that is the ratio
of the object’s mass to volume
D= m/v
 Reported as g/cubic cm
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Many common minerals have densities from 2-5
g/cubic cm
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Some metals have much higher densities (i.e. Au= 19.3
g/cubic cm)
We use known density values to help us determine
what an unknown sample might be
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Some minerals have special properties that
make them easy to identify!
Talc- Soapy feel
Graphite- Greasy feel
Gold- very malleable (easy to shape)
Magnetite and Hematite- Magnetic
Calcite- reacts with acid and produces double
refraction of light
Sulfur minerals- smell like rotten eggs