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| Crystals of
serandite, natrolite, analcime, and aegirine
from Mont Saint-Hilaire, Quebec, Canada. |
Minerals
In geology and mineralogy, a mineral or mineral species
is, broadly speaking, a solid chemical compound with a
fairly well-defined chemical composition and a specific
crystal structure, that occurs naturally in pure form.
The geological definition of mineral normally excludes
compounds that occur only in living beings. However some
minerals are often biogenic (such as calcite) or are
organic compounds in the sense of chemistry (such as
mellite). Moreover, living beings often synthesize
inorganic minerals (such as hydroxylapatite) that also
occur in rocks.
The concept of mineral is distinct from rock, any bulk
solid geologic material that is relatively homogeneous
at a large enough scale. A rock may consist of one type
of mineral, or may be an aggregate of two or more
different types of minerals, spacially segregated into
distinct phases.
Some natural solid substances without a definite
crystalline structure, such as opal or obsidian, are
more properly called mineraloids. If a chemical compound
occurs naturally with different crystal structures, each
structure is considered a different mineral species.
Thus, for example, quartz and stishovite are two
different minerals consisting of the same compound,
silicon dioxide.
The International Mineralogical Association (IMA) is the
generally recognized standard body for the definition
and nomenclature of mineral species. As of December
2020, the IMA recognizes 5,650 official mineral species
out of 5,862 proposed or traditional ones. |
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Characteristics of minerals
A mineral is a substance that usually |
- is an inorganic solid. (elemental
mercury is an exception)
- has a definite chemical make-up
- usually has a crystal structure;
some do not
- is formed naturally by geological
processes
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One recent definition is:
"A mineral is a homogeneous naturally occurring substance
with a definite but not necessarily fixed chemical
composition. Most minerals are solids with an ordered atomic
arrangement, and most are inorganic in the chemical sense of
that word".
Alternatively, a mineral is one listed as such by the
International Mineralogical Association.
Minerals and rocks
Minerals are different from rocks. A mineral is a chemical
compound with a given composition and a defined crystal
structure. A rock is a mixture of one or several minerals,
in varying proportions.
A rock has only two of the characteristics minerals have–it
is a solid and it forms naturally. A rock usually contains
two or more types of minerals. Two samples of the same type
of rock may have different kinds of minerals in them.
Minerals are always made up of the same materials in nearly
the same proportions. A ruby is a mineral. Therefore, a ruby
found in India has similar makeup as a ruby found in
Australia.
Formed in nature
Minerals are formed by natural processes. A few substances
with the same chemical composition as minerals can be
produced by living creatures as part of their shells or
bones. The shells of molluscs are composed of either calcite
or aragonite, or both.
Traditionally, chemicals produced by living things are not
considered minerals. However, it is difficult to see why an
organic substance should not be called a mineral if its
chemical nature and its crystalline structure is identical
with its inorganic twin. This issue is now under debate.
Minerals form in many ways. The mineral halite, which is
used as table salt, forms when water evaporates in a hot,
shallow part of the ocean, leaving behind the salt it
contained. Many types of minerals are made when molten rock,
or magma cools and turns into a solid. Talc, a mineral that
can be used to make baby powder, forms deep in Earth as high
pressure and temperature causes changes in solid rock.
The extraordinary thing is, that most minerals owe their
formation to life, or at least to the Great Oxygenation
Event. "Sturdy minerals rather than fragile organic remains
may provide the most robust and lasting signs of biology".
Solid
A mineral is a solid—that is, it has a definite volume and a
rough shape. Volume refers to the amount of space an object
takes up. For example a golf ball has a smaller volume than
a baseball, and a baseball has a smaller volume than a
basketball.
A substance that is a liquid or a gas is not a mineral.
However, in some cases its solid form is a mineral. For
instance, liquid water is not a mineral, but ice is. |
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Physical properties
These properties are used most often: |
- Crystal structure: see below
- Hardness: on the Mohs scale, a
ten-point scale running from the softest, talc to the
hardest, diamond.
- Lustre: appearance in light
- Colour
- Streak: colour of a mineral when it
has been ground to a fine powder. Often tested by
rubbing the specimen on an unglazed plate.
- Cleavage: how mineral splits along
various planes
- Fracture: how it breaks against its
natural cleavage planes
- Specific gravity: density compared
with water
- Any other properties
|
Crystal structure
The particles of ice that make up frost have smooth, flat
surfaces. These flat surfaces form because of the
arrangement of atoms in the ice, which is a mineral. Such an
internal arrangement is a characteristic of minerals. It is
the structure of a crystal, a solid in which the atoms are
arranged in an orderly, repeating three-dimensional pattern.
Each mineral has its own type of crystal structure. In some
cases, two minerals have the same chemical composition but
different crystal structures. For example, both diamond and
graphite consist of just one element—carbon. But the
arrangements of the carbon atoms in these two minerals are
not the same, so they have different crystal structures and
very different properties. Diamonds are extremely hard and
have a brilliant sparkle. Graphite is soft, gray, and dull.
In nature, a perfect crystal is rare. Most crystals have
imperfect shapes because their growth was limited by other
crystals forming next to them.
Chemical properties
Definite chemical makeup
Each mineral has a definite chemical makeup: it consists of
a specific combination of atoms of certain elements. An
element is a substance that contains only one type of atom.
Scientists can classify minerals into groups on the basis of
their chemical makeup. Though there are thousands of
different minerals, only about 30 are common in Earth's
crust. These 30 minerals make up most rocks in the crust.
For that reason, they are called rock-forming minerals. |
- Silicates are most common group. All
the minerals in this group contain oxygen and
silicon—the two most common elements in Earth's
crust—joined together. Silicates may include other
elements such as aluminium, magnesium, iron and calcium.
Quartz, feldspar, and mica are common silicates.
- Carbonates are the second most
common group of rock-forming minerals is the carbonates.
All the minerals in this group contain carbon and oxygen
joined together. Calcite, which is common in seashells,
is a carbonate mineral.
- Oxides include the minerals from
which most metals, such as tin and copper, are refined.
An oxide consists of an element, usually a metal, joined
to oxygen. This group includes haematite, a source of
iron.
- Sulphates contain the sulphate group
SO4. Sulphates commonly form in evaporites where highly
salty waters slowly evaporate, allowing sulfates and
halides to precipitate where the water evaporates.
Sulphates also occur where hot waters are forced through
the rock, as with geysers.
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There are many other mineral groups. |
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Some uses of minerals
People use minerals for many everyday purposes. Every
time people turn on a microwave oven or a TV, minerals
are being used. The copper in the wires that carry
electricity to the machine is made from a mineral. Table
salt or halite, is another mineral that people use in
their everyday life. |
- Graphite is used to make pencils
- Rock salt is used in cooking
- Mineral ores are the source of
metals.
|
 Kiddle: Minerals
Wikipedia: Minerals |
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