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| MINERAL IDENTIFICATION PROJECT
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Terms and Concepts
You've been out camping and picked up an unusual rock, someone brought back a stone carving from
Mexico, or you've chosen Geology 101 as an elective. You are now looking for a quick and painless
way to identify minerals. Well, you just found it!
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WHAT IS A MINERAL?
There are few general parameters that define minerals. First, minerals are a naturally
occurring, relatively uniform, mix of elements that results in a stable definable solid. Naturally
occurring means that a geologic process such as evaporation, weathering, volcanic eruption, or burial
at great depth created the conditions for mineral creation. The mix of elements, or chemistry,
is usually the defining property of a mineral and it over-rides most other properties. Second,
minerals (with few exceptions) are crystals. Crystals are a highly organized assemblage
of atoms whose internal organization is expressed by the development of external faces (facets) which
are unique to each mineral. Finally, the physical properties, such as chemistry, hardness,
density, luster and color, are highly consistent from one sample to another. This being said, minerals are from the earth, and are therefore a bit dirty in their chemistry and physical appearance. Where one sample will have large well developed purple crystals another will be green and have crystals too small to distinguish--but both can be the same mineral!
The following is a brief description of the physical parameters used by this search engine to
identifying mineral samples.
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HARDNESS:
Hardness is the property of a mineral to resist abrasion. Some minerals can be scratched easily
while others are almost impossible to scratch. Geologists developed a scale called the "Mohs
Hardness Scale" based upon the hardness of various minerals, each with a different resistance
to abrasion than other minerals in the scale. The scale runs from talc at 1, a very soft
mineral to diamond at 10, the hardest mineral. Scratch the surface of your sample with one
of the minerals from the Mohs kit and see if the surface is marked. If not, use progressively
harder minerals from the kit until it is marked. This will be the hardness of your sample.
To provide for some uncertainty in the hardness measurement, this search engine displays all minerals
with a .5 lower hardness to a 1.0 higher hardness than the number input.
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Mohs Scale
1
2
3
4
5
6
7
8
9
10 |
talc
gypsum
calcite
fluorite
apatite
orthoclase
quartz
topaz
corundum
diamond |
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Relative Scale
Less than 2
Between 2 and 3
Easily scratched 3 to 4.5
5 to 5.5
7
harder than 7
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Fingernails
Copper penny
Pocket knife
Window glass
Quartz
Steel file |
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DENSITY:
The density of a mineral is its mass per unit volume (g/cm3). This is generally compared
to an equal volume of water to come up with specific gravity (s.g.). One could make this comparison
by accurately weighing a mineral in grams and then sinking it in a graduated beaker of water where the
initial depth and the subsequent depth are measured. The mineral weight is then divided by the
volume of water displaced in cm3. Complex, but it can be done. You can make a relative
scale using sand and copper pennies. You eyeball the volume of your sample and compare it to
an equal volume of sand or pennies. If your sample weighs less than the sand then it is less
than 2.5 s.g., if more, then it's above 2.5 s.g. If it takes twice as much sand to equal the
weight of your sample then the sample is about 5 s.g. If it's more than 5 s.g. use the
pennies--they have a specific gravity of about 9. This search engine takes into account
some uncertainty in the s.g. of your sample, and it will select those minerals with approximately
the same s.g. you input.
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CLEAVAGE:
Cleavage is the property of a mineral to break along a definite plane or planes. Mica is an example
of a mineral with one perfect cleavage plane. Table salt, halite, is an example of a mineral with
3 perfect planes of cleavage. In looking through the references, you'll encounter various descriptions
of cleavage such as perfect, good, fair, poor--it is a judgmental scale and not easily quantified.
This database uses the following descriptions for cleavage:
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Cleavage
Perfect
Parallel
Basal
Poor
None
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3 or more good to perfect planes
2 good to perfect planes
one good to perfect plane
1,2, or more imperfect planes
No planes
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FRACTURE:
Fracture is the way a mineral breaks if it doesn't break along a cleavage plane. Minerals such
as quartz have smooth, curved fractures similar to a curved clam shell, other minerals break
differently. Minerals such as asbestos break into long fibrous or splintery fragments. Like
cleavage, the way something fractures is descriptive and variable.
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Fracture
Conchoidal
Splintery
Hackley
Uneven
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Smooth or rough curved break
Fibrous or splintery break
Rough surface with sharp edges
Rough surface
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LUSTER:
Luster is the property of the crystal surface to reflect light. Some transparent to translucent
minerals will have a glassy to adamantine (diamond like) reflection while other will have a waxy
to dull reflection. Some Opaque minerals will have a golden, coppery or silvery metallic luster.
Metallic luster is a definite indicator and should be used in the identification process. The
following descriptions are used in this database:
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Luster
Metallic
Submetallic
Glassy
Waxy
Dull
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Golden, coppery or silvery reflection
A somewhat metallic reflection
Adamantine or vitreous reflection
Oily, greasy silky or resinous reflection
Earthy, or non-reflective surface
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COLOR AND STREAK:
For some minerals, color is the defining property and a constant. For others, color is the result
of inclusions, impurities in the crystal structure, or the type of light available when viewing the
sample. Color is an important property and constant for minerals with a metallic luster, but in
transparent to translucent minerals it isn't. Finally on color, what is red to someone could
be scarlet, pink or brick red to someone else--color is generally a subjective property. Shades of
color such as dark red, red and pink are grouped together in the red color selection.
Streak is the finely powdered trace a sample leaves when scraped across an unglazed ceramic plate.
Although the color of your sample might be variable the streak is generally constant and therefore
more useful for mineral identification.
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CRYSTAL SHAPE:
Given adequate space to fully develop, minerals are driven by their internal chemistry to form into
various shapes. These shapes can be short and squat, long and slender, needlelike, booklike or like
grains of sand. Each mineral is unique in the faces it develops and the way these surfaces grow.
The following descriptions of crystal shapes were used in the database:
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Crystal Expression
Faceted
Prismatic
Tabular
Acicular
Microcrystalline
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Granular or stubby without elongation in any direction
Elongated in one direction only
Elongated in two directions
Needlelike or bladed crystals
Crystals too small to easily see
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