Transcript How Do We Identify Minerals? - School District of New London

How Do We Identify
Minerals?
Earth Science
Ms. Kurtzweil
Spring 2010
Identification by Careful
Inspection…
Color
► First
and most easily observed mineral property.
► Some minerals have very characteristic colors
that help identify them..
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Cinnabar
Malachite
Sulfur
Azurite
BUT… Color is also the least useful
property for mineral identification
►
Many different minerals
have similar colors
 Orthoclase feldspar, calcite,
others… milky white
►
Traces of impurities can
change a mineral’s color
 Traces of impurities can
change a mineral’s color…
Ex: Quartz / Corundum
+ Fe… amethyst / +Cr…
ruby
► + Ti… rose quartz / + Co,
Ti… sapphire
►
►A
mineral may change color when exposed to air
► Weathered surfaces may hide true color…
 So always inspect freshly-exposed surfaces!
Luster
► Luster:
how light is reflected from the surface
of a mineral
► Are either metallic or nonmetallic
► Metallic- shines like polished metal
 Galena
 Pyrite
►
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Nonmetallic lusters:
Glassy (vitreous)
 Quartz
►
Pearly
 Mica, talc
►
Waxy
 Sulfur, sphalerite
►
Brilliant (adamantine)
 Diamond
►
Earthy
 Gypsum
►
Others (greasy, oily, dull)
Crystal Shape
► When
minerals have
enough time and space
to form, their atoms or
ions always arrange
themselves into patterns
► Form 1 of 6 basic crystal
shapes
 Flat-faced
 Regularly shaped
 RARE to find
Identification by Simple
Tests…
Streak
► Streak:
the color of a
mineral in powdered form
► Although the color of a
mineral may vary, the streak
rarely does. Much more
reliable!
► Examples:
 Iron pyrite (brass-yellow)
streak: greenish-black
 Hematite (brown,red,silver)
streak: reddish brown
► To
test streak:
 Rub specimen across an unglazed ceramic tile, called a
streak plate (white or black)
► Streak
rule of thumb
 Metallics: streak will be at least as dark as hand
specimen
 Nonmetallics: streak is usually colorless or white
Cleavage
► Cleavage:
a mineral’s
tendency to break
evenly or separate
along a flat surface.
► Ability is related to the
type of bond in atomic
structure- will separate
at weak bonds
► Can be observed in
tiny grains; therefore
very useful
► Micas
(muscovite,
biotite)
 1 perfect plane of
cleavage
 Parallel sheets
► Calcite,
galena
 3 planes of cleavage
 Right angles
► Fracture:
when minerals with no cleavage
planes break unevenly into curved or
irregular pieces
 Uneven/irregular
►Ex.
Cinnabar, some quartz
 Splintery/fibrous
►
asbestos
 Conchoidal- like broken glass
►Obsidian
Density
►
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Depends on the kind of atoms
present (more massive?) and how
closely packed they are
Heft
 Galena “feels” heavier than quartz
►
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Density is more specific: d=m/v
Most common minerals in Earth’s
crust range from
Best for identifying heavier metals
(Au, Pb, U, Ag) which range from
Hardness
► Hardness:
the ability of a
mineral to resist
scratching
► Mohs’ Scale of Hardness
 Used as a practical test
for determining a
mineral’s hardness
 1 (softest) Talc
 10 (hardest) Diamond
Mohs Knows Best
Don’t confuse a streak with a scratch!
 Rub away streak to feel and look for the scratch
► Ask:
 Which is the hardest mineral on the scale that it can
scratch?
 Which numbers does it fall between?
 If neither scratch each other, they have the same
hardness
► Graphite and diamond are both pure C
 Graphite: 1, Diamond: 10
 Difference is in their bonds!
►
Special Properties…
Magnetism
► Mineral
can be picked
up by a magnet, or will
pick up small metal
objects
► Examples:
 Magnetite
 Lodestone
Fluorescence/Phosphorescence
► Fluorescence:
ability to
glow under ultraviolet
(UV) light
 Some fluorite, calcite
samples
► Phosphorescence:
ability to glow after UV
light is cut off
 Some willemite,
sphalerite samples
Double Refraction
► Refraction:
the
bending of light rays
as they pass through a
translucent substance
► Double refraction: the
splitting of light rays
into two parts, causing
a double image
► Calcite
Effervescence
►A
mineral is said to
display effervescence if
it fizzes in a chemical
reaction with cold,
weak HCl
► Limestone and Marble
 Calcite (CaCO3) is the
principle mineral
 Bubbles released are
carbon dioxide
Radioactivity
► Certain
atoms have unstable
electron and/or nuclei
arrangements
► An unstable nucleus becomes
stable over time by releasing
particles and energy (radiation)
► Most common elements:
Uranium and Radon
► Minerals: carnotite and
uraninite found in U