A mineral is a naturally occurring homogeneous solid of definite chemical composition and ordered atomic arrangement. It is usually formed by inorganic processes.
More than 5000 mineral species have been described and more than 100 new minerals are described every year. To describe a new mineral, the structure and composition must be described.
Most of these 5000 minerals are rare, so that only about 200 are common enough to make up macroscopic rocks. Of these, the most abundant 50 make up 99.9% of the Earth's crust.
Some examples of minerals that are familiar to you are quartz (SiO2, silicon dioxide), calcite (CaCO3 calcium carbonate), pyrite (FeS2, iron sulfide), gypsum (CaSO4 . 2 H2O), gold (Au), silver (Ag), copper (Cu), diamond (C), graphite (C), garnet (Mg3Al2Si3O12), ice (H2O). Less familiar, perhaps, are apatite (Ca5 (PO4)3OH) (teeth and bone), olivine (Mg2SiO4) (the green mineral that makes up much of the upper mantle, gem variety: peridot), pyroxene (MgSiO3) (the other mineral in the upper mantle), muscovite (white) mica (KAl2(AlSi3)O10(OH)2), and feldspar (Na,K)AlSi3O8 or CaAl2Si3O8.
A crystal is a three-dimensional periodic array of atoms. Most solids are crystalline, but some, like glass, opal, amber, wood, and coal, are not and are said to be amorphous.
The basic repeat unit of a crystal or mineral is the unit cell.
Crystals have complex symmetries. Ice, apatite, and graphite are hexagonal (six-fold symmetry); quartz and calcite are trigonal (three-fold symmetry); diamond, garnet, gold, pyrite are cubic (three four-fold axes), olivine is orthorhombic; micas are monoclinic; and feldspars are triclinic.
A given compound like SiO2 may occur as different crystalline forms. If the crystalline structures of the two forms are different, they are said to be polymorphs.
SiO2 may occur as the mineral quartz, or, at high temperatures quartz transforms to tridymite, and then to cristobalite. If a rock containing quartz is subducted into the mantle, the quartz transforms first to coesite at a depth of about 100km, then to stishovite at a depth of about 250km. These minerals are said to be polymorphs of quartz. Graphite and diamond are polymorphs of carbon. Pyrite and marcasite are polymorphs of FeS2.
Conversely, chemical elements that have similar chemical properties may substitute for one another in a given crystal structure so that the same structure may occur with different compositions. This is called isomorphism. An example is albite (NaAlSi3O8) and orthoclase (KAlSi3O8) feldspars. In the feldspar crystal structure, Na (sodium) and K (potassium) are alkalis and can substitute freely for one another, so that all compositions between albite and orthoclase may exist.
We can use some simple observations of physical properties of minerals to distinguish or identify the most common minerals.
Hardness is the ability of a mineral to resist scratching. We use the Mohs' hardness scale for field testing:
1. Talc 4. Fluorite 7. Quartz 2. Gypsum 5. Apatite 8. Topaz 3. Calcite 6. Orthoclase 9. Corundum 10. Diamond
Luster is the reflectivity of a mineral: metallic or non-metallic. Of the non-metallic minerals some may be glassy (vitreous), dull (earthy), or adamantine (bright with many internal reflections) as in diamond.
Color is simply the color of a mineral in hand specimen, but tiny amounts of certain elements like iron can strongly color a hand specimen. So the color of a powdered mineral is considered much more diagnostic. An easy way to do this test is to examine the color of the mineral's streak when scratched on a porcelain plate.
The external form or shape of crystals may be the result of fracture or of growth. When a crystal grows in a liquid (magma or aqueous fluid), its rapid growth directions become points and its slow growing directions become faces.
If a crystal has a plane of weak chemical bonds it can break along perfect planes. This is called cleavage. Mica has a single perfect cleavage whereas calcite has three. Quartz has no cleavage and breaks like glass (conchoidal fracture).
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