MINERALS
Characteristics and Properties
Mineral Characteristics
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What is a mineral?
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– Naturally occuring
• Not man-made (synthetic) or
biologically produced.
– Pearls, styrofoam, charcoal are
not minerals
– Inorganic
• Not formed from processes involving
organisms (living or once living)
– Solid
– Has definite chemical composition
• An element or compound
– Has an orderly arrangement of atoms
• Crystalline structure-that is-atoms
have repetitive patterns and internal
structures are distinct.
• Geometric solids with smooth
surfaces (crystal faces)
Identifying Characteristics of
Minerals
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Color
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Color by itself IS NOT sufficient to
identify a mineral
Luster
•
How the material reflects light
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Hardness
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How easily the mineral can be
scratched
Can be compared to the hardness of
other minerals by using the Mohs scale
Streak
•
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The powder form of the mineral left on a
porcelain plate (must be softer than the
porcelain)
Cleavage
•
The way the mineral splits along flat
surfaces
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Metallic-shiny. Ex: silver, copper, etc
Nonmetallic-Pearly or cloudy, dull
Determined by the arrangement of the
atoms
Not all minerals have cleavage
Density
•
Ratio of mass to volume
–
Determined by the mass of the atoms
and how close they are
Formation of Minerals
•
How are minerals formed?
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One way is the cooling of magma
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Thermal energy is lost; atoms
migrate together and form different
compounds
The elements present and the
amounts determine the kind of
minerals
Different crystal structures are
formed
If the magma cools slowly, large
crystals are formed. Different
minerals form at different
temperatures. Heavier minerals such
as magnetite sink and lighter ones
float.
Minerals such as quartz and calcite
form late in the cooling process and
are known as hydrothermal minerals.
In the last few years, hydrothermal
vents have been formed on the
ocean floor. In these areas, sea water
filters into the hot crust and is
heated to 400 degrees C. The hot
water then reacts with the crust and
becomes a metal bearing liquid.
When it returns to the cooler sea
floor, it deposits minerals, including
iron, copper and zinc sulfide.
--Minerals can precipitate out of a
solution
When water is saturated with dissolved solids
and can’t hold any more, the excess falls out
of the solution. An example of this is the
manganese nodules on the ocean floor
--Minerals can form by evaporation
•
Minerals such as salt, gypsum and calcite
(calcite forms in two ways) are formed from
sea water when it evaporates. This happens
in warmer parts of the world where the sun's
heat evaporates the water and leaves the
minerals.
Other ways that minerals are formed:
–
Some minerals are formed from the
weathering of rocks.
• Chemical changes are caused by
atmospheric oxygen, water and
acid rain. Such action can change
feldspars to kaolin and pyrite
(fools gold) into a brown iron ore
called limonite
–
And lastly some minerals are formed
when rocks are metamorphosed, that
is subjected to heat and pressure.
• Minerals formed in this way
include garnet and mica.
Mineral Groups
• Minerals that contain oxygen and silicon are
called “silicates”. These two minerals combine
to form most of the minerals in the earth’s crust.
• They are the most abundant single minerals in
the earth’s crust (oxygen-46.6%; silicon-27.7%)
• More than half of the minerals in the earth’s
crust are feldspars which is a silicate
• Some other examples of silicates include: talc,
quartz, mica, topaz, hornblende, garnet, zircon
•
Uses of Minerals
Gems
– Rarity and beauty makes them valuable
– Gemstones used for jewelry are cut and polished and sometimes don’t resemble
the raw form of the mineral
• May have a crystal structure that allows it to be cut in facets
• May have the addition of another mineral that gives it a brighter color
•
Ores
– A mineral is an ore if it contains a substance that can be sold for profit
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Hematite is the ore of iron
Bauxite is the ore of aluminum
Copper comes from chalcopyrite ore
Rutile is the ore that titanium comes from
– Titanium is valuable because of its strength and low density (lightness)
– Ore deposits are formed when fluids travel through weaknesses in rocks, such
as fractures and cracks.
– The minerals dissolved in the fluids are left behind when the liquid evaporates,
forming “vein deposits
•
Mining
– Ores are only profitable if the cost of mining them is less than the value of the
material being mined
• Waste rock has to be removed
• This can be expensive and harmful to the environment
Physical Appearance
• Color cannot be used exclusively to identify
minerals. For example the color of turquoise can
vary from blue to green
• Minerals may have varying degrees of
transparency, which is the ability of light to pass
through a substance—quartz can be transparent,
but can contain flaws that make it translucent.
This is related to Luster.
• Luster is definitely an identifying physical
characteristic
• It describes how light reflects from the surface of
the mineral. Describers might include:
– Metallic, waxy, pearly, earthy, dull,
glassy (vitreous), silky
Hardness
• This is how easily a mineral can be scratched and is
definitely an identifying characteristics. In order to be
scratched by an object, the mineral must be softer than the
object doing the scratching.
• The Mohs scale is a system of comparing the hardness of a
list of 10 minerals (The Giant Cat Found a Foolish Quail that
Couldn’t Dance)
– Developed by Frederich Mohs
– It lists them in order from 1 (softest) to 10
– It gives a list of common objects and their hardness
• If the mineral scratches the common object but the object won’t scratch
the mineral, the mineral is the hardest
• Moh's Scale
Streak
• Streak is the color of the powder form of the
mineral
•
A streak plate is used to do this test. It is a
piece of unglazed porcelain—which has a
hardness of 7 on the Moh’s scale. A streak of the
mineral is left on the porcelain when it is rubbed
across with the mineral. The mineral must be
softer than the streak plate if this is to work.
•
Some minerals leave a certain color streak,
which is an identifying characteristic.
– An example of this is hematite, which although it is a
black mineral, leaves a red streak
Cleavage and Fracture
• When a mineral breaks it does so either by
fracturing or by cleaving. Crystal cleavage is a
smooth break producing what appears to be a
flat crystal face. Here are a few rules about
cleavage. First cleavage is reproducible,
meaning that a crystal can be broken along the
same parallel plane over and over again. All
cleavage must parallel a possible crystal face.
• Fracture describes the way a mineral breaks
and is different from cleavage. A fracture might
be splintery, conchoidal (like glass) , jagged or
earthy (like a ball of clay)
Common Minerals
• There are more than 4000 named minerals, but
most rocks are made up of just a small
number—these are the “rock forming” minerals
• Central Florida is the heart of the US phosphate
industry and the leading producer of phosphate
in the world. Phosphate is used to make
fertilizer
– Phosphate is a profitable industry in Florida
– Mining for phosphate also produces toxic substances
such as uranium and arsenic because the phosphate
is buried under these substances. The by-products
can’t be returned to the mine because they are a
source of pollution