Minerals – page 1
Minerals
Introduction
Most people are familiar with the commonly applied classification of all physical
entities into the three categories of animal, vegetable and mineral. We now know that
these subdivisions are not in the strictest sense discrete. Some organisms exist that
combine animal locomotion with plant photosynthesis. Others, such as viruses, grow and
reproduce under some conditions, and crystallize like minerals under other conditions.
However, the general implications of this fundamental breakdown are valid: minerals are
non- living and occur in many species as do plants and animals.
The study of minerals is the area in geology most commonly translated into a
satisfying hobby for the layman. Indeed, one need not be a full- fledged geologist to
apprecia te the aesthetic and intrinsic value of minerals, and to enjoy identifying and
collecting them. The primary objective of this unit is to introduce minerals selected from
a kingdom of several thousand species, and to define the various physical criteria used to
distinguish one mineral from another.
Definition
In order to be considered a mineral, a substance must satisfy four criteria:
1. It must be inorganic
2. It must be naturally occurring
3. It must have a definite chemical make-up
4. It must be crystalline in nature
Inorganic:
Minerals are not, and never were, living. The only “gray area” is carbon. Carbon
is product of organic processes, but remains accepted as a mineral by convention.
Naturally Occurring:
Minerals are not man- made, but happen naturally. Man- made substances may be
chemically identical to minerals, but area not classified as minerals. Synthetic
rubies, emeralds sapphires, and other gems may be flawless, but have no
investment value.
Definite Chemical Make-up:
The basic identity of a mineral is its chemistry. Each species contains certain
elements in specific proportions. For example, quartz has the formula Si0 2 . This
means that for every type of quartz, there is one silicon atom for every two
oxygen atoms.
Crystalline in nature:
In minerals, the atoms are organized in orderly networks at specific distances and
angles to one another. Glass is not a mineral because it lacks this ordered internal
structure.
Minerals – page 2
Physical Properties
Each mineral species has a discrete chemistry, and can therefore be identified
using certain chemical tests. This is not very practical for fieldwork, and mineralogists
have established a series of physical properties, which will suffice to distinguish mo st
minerals from each other.
In mineral identification, not all of these properties apply to every specimen.
Some minerals have one specific test that serves to identify them quickly. For example,
some minerals have an unusual smell, while others break in a certain way. Often the tests
are used together to form and overall identification of a mineral. Students should be able
to perform simple tests to help determine the identification of each mineral.
I.
Hardness
Hardness is a relative scale, evaluated on a level of 1 to 10, with 1 being very soft,
to 10, which is the hardness of a diamond, the hardest of all minerals.
Each mineral on this scale can scratch a mineral with and equal or lower number,
but not one with a higher number. For example, calcite can scratch calcite,
gypsum, talc, or your fingernail, but nothing with a harness greater than three. All
minerals have a hardness somewhere in the 1 to 10 range, but may not be a whole
number. Sphalerite can scratch calcite(3) but not fluorite(4), so it is a 3.5.
Minerals with a hardness of less than 6 will not withstand prolonged wear as
jewelry. Many gemstones are variants of quartz, and have a hardness of 7.
When testing for hardness, make a small scratch, about ¼” long, to avoid
unnecessary damage to the samples. Wipe your finger across the scratch to be sue
it isn’t merely a powder line made by the other sample. Use firm pressure, but
you don’t need to be excessive. When testing on a glass plate, always place the
glass flat on the table to avoid injury.
1
2
2¼
3
3½
4
5
5½
6
6
7
8
9
10
TALC
GYPSUM
Average fingernail
CALCITE
Copper penny
FLUORITE
APATITE
Steel nail, knife blade
Glass plate
ORTHOCLASE
QUARTZ
TOPAZ
CORUNDUM
DIAMOND
Moh’s Hardness Scale for
Minerals
Minerals – page 3
II.
LUSTER:
Luster is the appearance of a mineral’s surface in reflected light.
Metallic Luster:
These minerals have a metallic glint, often silver or gold, and reflect
considerably. Metallic minerals are always opaque, no matter how thin they are
sliced.
Non-Metallic Luster:
These fall into several subcategories, the most common are:
Adamantine: Brilliant, like a polished diamond
Vitreous: Glassy, like glazed porcelain or quartz
Resinous: Like resin, Sphalerite is an example
Pearly: Like pearls, Talc, Alabaster Gypsum
Silky: Results from parallel fibers; asbestos, Satin Spar Gypsum
Earthy: Dull, little reflection
III.
COLOR:
This would seem to be a simple attribute to deal with, and yet it really does not
work very well for mineral identification. Colors are subjective to the observer
and are unreliable for many specimens. Quartz, fluorite, calcite, and other
fundamentally colorless minerals may take on the full spectrum of hues
depending in trace element contamination.
Smaller crystals often look lighter in aggregate. In some specimens a thin surface
coating of exotic material may be misleading. Color is most dependable for the
metallic minerals.
For the purpose of this class, light and dark are the only color categories we will
use. Light colors are: white, clear, pink, light gray, salmon, and yellow. Dark
colors are: dark gray, brown, black, red, and green. Metallic minerals are
divided into silver and gold.
IV.
STREAK:
This is the color of the mineral when powdered. Streak is generally constant,
even when the color of the uncrushed mineral varies. Most non- metallic minerals
have a non-diagnostic white streak, but metallic samples have a dependable
streak. Specimens may be crushed in a mortar, but the usual practice is to drag
them across the abrasive surface of an unglazed tile. If the specimen is harder
than the streak plate (5 ½ to 6) then it will not leave a streak.
Minerals – page 4
V.
CLEAVAGE
This is the tendency of a mineral to break along certain planes determined by
internal atomic arrangement and bonding strength.
Common types of cleavage are:
1. ONE DIRECTION (perfect cleavage): This looks like pages in a book and
thin sheets can be pulled off easily.
2. TWO DIRECTIONS: This is often difficult to see, but usually two sides
come to an angle greater than 90o .
3. THREE DIRECTIONS: Three directional cleavages fall into two categories,
either cubic, where all the angles are exactly 90o , or non-cubic, where the
angles can be greater or less than 90o . Cubic looks like a box, or cube, while
non-cubic looks like a rhomboid.
4. FOUR DIRECTIONS: These look like two pyramids stuck together, base to
base.
There are many other types of cleavage, but these are the only ones we will be
discussing in class.
VI.
FRACTURE
This is the non-planar breakage of minerals. There is no noticeable cleavage
planes. Fracture may be described as rough, splintery, or conchoidal (breaks like
glass, into curves and/or circles).
VII.
CRYSTAL FORM:
Most minerals will take on a definite external geometric form when unobstructed
growth is allowed. This form is an expression of their orderly internal atomic
arrangement or crystallinity. Whole crystals and crystal faces are therefore and
aid to identification. A complete study of crystallography is beyond our scope in
this class.
Minerals – page 5
VIII. MISCELLANEOUS:
Other properties can be specific to one mineral, or are less common and only
diagnostic for a few specimens.
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Natural Magnetism: magnetite
Feel: talc fees “soapy” or “greasy”
Specific Gravity: galena is heavy, talc is light
Reacts with acid: calcite will fizz when it comes into contact with a 10%
HCL solution
Tenacity: reaction under stress; brittle (galena), flexible (talc), elastic
(muscovite), malleable (native copper)
Double refraction: calcite will give a double image when looked through
Fluorescence: emits color under ultraviolet light
Odor: sulfur smells like rotten eggs
Taste: halite tastes like salt