What is a mineral?
When you see a glittering crystal in our Museum you
would probably say it was a mineral - and you'd almost
certainly be right. But can you say what a mineral really
is?
Defining a mineral
A mineral is a naturally occurring inorganic solid, with a
definite chemical composition, and an ordered atomic
arrangement. This may seem a bit of a mouthful, but if
you break it down it becomes simpler.
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Minerals are naturally occurring
They are not made by humans
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Minerals are inorganic
They have never been alive and are not made up from plants or animals
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Minerals are solids
They are not liquids (like water), or gases (like the air around you)
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Minerals have a definite chemical composition
Each one is made of a particular mix of chemical elements
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Minerals have an ordered atomic arrangement
The chemical elements that make up each mineral are arranged in a particular way this is why minerals 'grow' as crystals
The tricky bits ... atoms, molecules and the rest
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There are around four thousand different minerals in the world. They are all naturally
occurring inorganic solids. Each mineral is defined by its particular chemical composition
and crystal structure. To explain what a mineral is properly, we have to introduce you to a
bit of chemistry.
Minerals are made up of chemical elements. A chemical element is a substance that is
made up of only one kind of atom. Have you heard of oxygen, hydrogen, iron,
aluminium, gold and copper? These are all chemical elements.
But what is an atom?
An atom is the smallest unit of any chemical element. They are the building blocks that
make up each chemical element, and are far too small to see with the naked eye. Imagine
a small piece of copper, for example. Even the tiniest piece of copper is made up of
billions and billions of copper atoms.
There are 103 types of atoms, and because each chemical element is made up of only one
kind of atom there are 103 chemical elements.
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Billions and billions of copper atoms stack together to form a piece of copper
Each mineral has a fixed chemical composition. Some minerals are made up of just one
chemical element - they contain only one type of atom. Native copper is made up of
copper atoms only. Most minerals are chemical compounds - they contain atoms of more
than one chemical element
So what is a chemical compound?
Put it this way. Just as chemical elements are made up of atoms, a chemical compound is
made up of molecules. Each molecule in a chemical compound is made up of two or
more different atoms connected together.
Halite is a chemical compound. It is also called sodium chloride, but you would probably
know halite better as rock salt - yes, the kind you put on your fish and chips. Each
molecule of halite contains one atom of an element called sodium connected to one atom
of an element called chlorine. Halite always contains just as many sodium atoms as it
does chlorine atoms: this 'formula' does not vary - halite, like all other minerals has a
definite chemical composition.
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Chlorine and sodium atoms will connect together to make halite molecules
The atoms in a mineral are arranged in an organised 'atomic structure'. They connect
together to form molecules, and the molecules stack together in a regular pattern to form
a crystal. The shape of a crystal depends on the way the molecules are stacked up inside
it.
There is a set way in which the sodium atoms "connect" to the chlorine atoms to make a
halite molecule, and these molecules can stack together to form a halite crystal.
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Halite molecules stack together to form halite crystals - this is the salt we eat everyday
All the properties of a mineral - its crystal shape, hardness, colour, lustre - depend on
which chemical elements it is made of and how the atoms of these elements are arranged
inside it.
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Identifying Minerals
Color - The color of a mineral is one of its most obvious attributes, and is one of the
properties that is always given in any description. Color results from a mineral’s chemical
composition, impurities that may be present, and flaws or damage in the internal structure.
Unfortunately, even though color is the easiest physical property to determine, it is not the
most useful in helping to characterize a particular mineral. Many minerals come in a rainbow
of colors.
Streak - The color of a mineral when it is powdered is called the
streak of the mineral. Crushing and powdering a mineral eliminates
some of the effects of impurities and structural flaws, and is
therefore more diagnostic for some minerals than their color. Streak
can be determined for any mineral by crushing it with a hammer, but
it is more commonly (and less destructively) obtained by rubbing the
mineral across the surface of a hard, unglazed porcelain material
called a streak plate.
The color of the powder left behind on the streak plate is the
mineral's streak. The streak and color of some minerals are the same.
For others, the streak may be quite different from the color.
Luster - The luster of a mineral is the way its surface reflects light. Most terms used to describe
luster are self-explanatory: metallic, earthy, waxy, greasy, vitreous (glassy), adamantine (or
brilliant, as in a faceted diamond). It will be necessary, at least at first, only to distinguish
between minerals with a metallic luster and those with one of the non-metallic lusters. A metallic
luster is a shiny, opaque appearance similar to a bright chrome bumper on an automobile. Other
shiny, but somewhat translucent or transparent lusters (glassy, adamantine), along with dull,
earthy, waxy, and resinous lusters, are grouped as non-metallic.
Hardness - Hardness is defined by how well a substance will resist scratching by another
substance. For example, if mineral A scratches mineral B, and mineral B does not scratch
mineral A, then mineral A is harder than mineral B. If mineral A and B both scratch each other,
then their hardness is equal. A scale to measure hardness was devised by Austrian mineralogist
Frederick (Friedrich) Mohs in 1822, and is the standard scale for measuring hardness. The scale
consists of numbers one through ten; 1 being the softest and 10 being the hardest. Each number
represents a different mineral - each harder than the previous. The 10 minerals are:
1. Talc
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Feldspar
7. Quartz
8. Topaz
9. Corundum
10. Diamond
All conceivable minerals fit in this scale, since Talc is the softest known mineral and Diamond
the hardest. To demonstrate how to use the scale, understand the following example: Suppose a
mineral scratches Fluorite, but not Apatite, then it has a hardness between 4 and 5.
Several common household items have a fixed hardness, and can be used to test for hardness:
Fingernail
2½
Penny
3 - 3½
Iron Nail
4½
Glass
5½
Steel file
6½
Streak plate (floor tile) 6½ - 7
Hardness is almost always rounded off to the nearest half number.
Specific Gravity - The specific gravity of a substance is a comparison of its density to that of
water. Imagine a gallon bottle filled with water, a second filled with feathers, a third filled with
lead weights. There are equal volumes of material present, but the bottle with the feathers will
weigh less than that containing water; the bottle with lead weights will weigh the most. In order
of increasing specific gravity, these materials would be: feathers, water, lead. Specific gravity
can be measured precisely, or estimated by a comparison, as above. To compare the specific
gravity of any two minerals, simply hold a sample of one in your hand and "heft it," i.e., get a
feeling for its weight. Then heft a sample of the other that is approximately the same size. If
there is a great difference in specific gravity, you will detect it easily. It is often sufficient to note
whether a mineral's specific gravity is significantly higher or lower than that of other minerals.
Heft each of the specimens in your mineral set. Which ones have a high specific gravity? A low
specific gravity?
Crystal Shape - When minerals form in environments where they can grow without interference
from neighboring grains, they commonly develop into regular geometric shapes, called crystals,
bounded by smooth crystal faces. The crystal form for a given mineral is governed by the
mineral's internal structure, and may be distinctive enough to help identify the mineral. For
example, quartz forms elongated, six-sided prisms capped with pyramid-like faces; galena and
halite occur as cubes; and garnets develop 12- or 24-sided equidimensional forms. Interference
from other mineral grains during growth may prevent formation of well-formed crystals. The
result is shapeless masses or specimens that developed only a few smooth crystal faces. This type
of specimen is much more common than well-formed crystals.
Cleavage & Fracture – Cleavage and Fracture are minerals properties that describe the way a
mineral breaks when stressed.
Cleavage describes the way a mineral breaks along its plane of weakness. These planes of
weakness can be described as where the atomic bonding between atoms in the crystal structure is
weak at those points causing the mineral break when stressed much like a stack of papers when
drop will spread off in individual sheets.
There are five basic types of cleavage.
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One Directional- One Directional Cleavage peels in layers much like taking individual
cards off a deck. Mica is a good example of this type of cleavage.
Two Directional - Two Directional Cleavage breaks along two directions but not at right
angles. Students seem to have the hardest time describing or identifying this cleavage. A
good rule of thumb to determine this type if your not sure is to eliminate the other types
of cleavage which are easier sometimes to see and what is left is the two direction.
Feldspar is a good example of this type of cleavage.
Three Directional Not at Right Angles - Three Directional not right angles is often very
easy to see. The mineral seems to have three dimensional boxes that would break if
dropped. These boxes are leaning on their sides much like a Rhombus. Calcite is a good
example of this type of cleavage.
Cubic Cleavage - Cubic Cleavage is three directional cleavage at right angles. This
cleavage can be easily recognize in that in breaks into perfect squares or cubes. A good
example of this type of cleavage is the mineral Halite or "Salt" as it is commonly called.
Octahedral Cleavage - Octahedral Cleavage is complex cleavage system where two foursided pyramids are stack on top of each other. This cleavage is often hard to see in some
mineral samples containing it. A good example of this type of cleavage is the mineral
Fluorite.
Fracture is a mineral property where the atomic bonding between atoms in the crystal structure
is perfect with no weakness. When these minerals are stressed they shatter making no two pieces
truly the same. There are two basic types of fractures, Conchoidal and Non-Conchoidal.
Conchoidal Fracture - Conchoidal Fractures are easy to spot. They have a circular pattern in their
break are much like broken bottle glass. Obsidian is a good example of this mineral property.
Non-Conchoidal Fracture - Non-Conchoidal Fracture minerals are also easy to identify. They
have no directional cleavage planes or swirls in there structure. A good example of this is the
mineral Quartz.
Other Properties - There are a few other tests that can be used to differentiate one or more
common minerals. Some of these should be used with great CAUTION!
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Magnetism - A few minerals are attracted to a magnet or are themselves capable of
acting as magnets (the most common magnetic mineral is magnetite). Because these are
so rare, this property helps narrow the possibilities drastically when trying to identify an
unknown specimen.
Feel - Some minerals, notably talc and graphite, feel greasy or slippery when you rub
your fingers over them. The greasiness occurs because bonds are so weak in one direction
that your finger pressure alone is enough to break them and to slide planes of atoms past
neighboring atomic layers.
Taste - Geologists use as many senses as possible in describing and identifying minerals.
Taste is one of the last tests to be conducted, because some minerals are poisonous. Some
minerals taste salty-most notably halite (salt). Sylvite, a mineral similar in all other
properties to halite, tastes bitter. Taste is thus a diagnostic property because it
distinguishes between these minerals. NEVER TASTE A MINERAL UNLESS
INSTRUCTED TO!
Reaction with Dilute Hydrochloric Acid - This is actually a chemical property rather
than a physical attribute of a mineral. Minerals containing the carbonate anion (C03)2effervesce ("fizz") when a drop of dilute hydrochloric acid is placed on them. Carbon
dioxide is liberated from the mineral and bubbles out through the acid, creating the fizz.
This test is best performed on powdered minerals. Calcite (calcium carbonate) will
effervesce readily in either massive or powdered form, but dolomite (calcium-magnesium
carbonate) reacts best as a powder.
Flourescence - Fluorescent minerals emit visible light when exposed to ultraviolet
light. “Glow in the dark”
References:
http://www.oum.ox.ac.uk/thezone/minerals/define/index.htm
http://geology.csupomona.edu/mineral/minerals.htm