A mineral has the following five properties:
The combination of properties 4 and 5 give each mineral a unique set of physical properties
Two minerals may share exactly the same composition, such as graphite and diamond, but because the arrangement of atoms is different, their physical properties (hardness and color for example) are very different.
The physical properties of minerals include:
Minerals are grouped according to the anion, or complex anion of their composition. The important mineral groups are presented in Table 3-3 of your text, on page 42. Notice that the suffix "ate" indicates that the anion is a complex anion that included oxygen. The two most important mineral groups by abundance in rocks are the carbonates (CO3) and the silicates (SiO4)
The carbonates minerals are formed at the Earth's surface through chemical or biochemical activity. Many marine organisms make their skeletal parts from calcite. This material accumulates to form the sedimentary rock limestone. The rock-forming carbonates are calcite and dolomite. Carbonates "fiz" when hydrochloric acid is placed on them. The acid reacts with the mineral releasing carbon dioxide gas.
The silicates are the most abundant mineral group. The crust and mantle of the Earth, about 80% of the volume of the planet are predominantly made of silicates.
The complex anion of silicates is a tetrahedron of four oxygen atoms surrounding one silicon atom, connected with strong covalent bonds.
In silicate minerals, the tetrahedra are packed together, so that the entire mineral crystal can be thought of as a pile of tightly packed oxygen atoms with silicon atoms between some of the oxygens, and other metallic atoms occupying other spaces between the oxygens. Because silicon atoms can share oxygen atoms, there are a variety of ways to build silicate structures. This gives rise to a set of basic groups of silicate minerals:
In isosilicates, each tetrahedron is independent. They are bonded together ionically by metal ions, often iron and magnesium, between the tetrahedra. Olivine, one of the rock-forming silicates, is an isosilicate. Olivine is a good example of the variability in the chemical composition of minerals. Olivine is an ferromagnesian silicate. In olivine, the ratio of iron to magnesium can vary from 0 to 1. In otherwords, some olivines might contain no iron, only magnesium. Other olivines might contain no magnesium, only iron, while most olivines contain a mixture of iron and magnesium.
In chain silicates, each silicon shares either two, or three oxygen atoms with adjacent silicon atoms. Thus each tetrahedron is not distinct, and they are linked together in strong, covalently bonded chains. The chains are then held together by ionic bonds. Pyroxenes are a single-chain silicates. Amphiboles (including hornblende) are double-chain silicates.
In sheet silicates, each silicon shares three oxygens with adjacent silicon atoms resulting in strong, covalently bonded sheets. The sheets are then held together with ionic bonds. Micas are sheet silicates. Micas split apart along planes of ionic bonding with K atoms. Biotite mica contains Fe and Mg giving it a dark color, while muscovite contains only K. Clay minerals are sheet silicates, but the crystals are too small to see with the naked eye. Micas and clays are also aluminum silicates.
In framework silicates, each silicon shares all four of its oxygen with adjacent silicon resulting in a three dimensional framework of strong covalent bonds. The simplest framework silicates is quartz which contains only silicon and oxygen. The feldspars are framework aluminum silicates. Since aluminum occupies the middle of some of the tetrahedra, other metals, namely K, Na and Ca are needed to balance the charge of the crystal.
Many of the chain, sheet and framework silicates, including feldspars, micas and clays, are aluminum silicates. Since the aluminum ion and the silicon ion are nearly the same size, aluminum can occupy the center of a tetrahedron. In aluminum silicates, some of the tetrahedra have silicon in the center, others have aluminum.
Ferromagnesian silicates contain iron and magnesium. Since these two metal ions are nearly identical in size, they easily substitute for one another in the crystal structure. Ferromagnesian silicates are usually dark in color. Olivine, pyroxene, amphibole and biotite mica are ferromagnesian silicates.
The chemical composition of silicates is related to their structure. As more oxygen is shared, fewer metallic ions are needed to balance the charge on the crystal. Thus the relative amounts of oxygen, silicon and aluminum in the mineral increase, as the amount of iron, magnesium and calcium decrease. Silicate minerals rich in iron, magnesium or calcium are called mafic. Silicate minerals rich in silicon, aluminum and potassium, and relatively poor in iron, magnesium and calcium are called felsic. Mafic and felsic can be used as absolute or relative terms for silicate composition.
The rock forming minerals are the minerals that make up the bulk of the rocks of the Earth's crust and mantle. They are:
Silicates:
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Carbonates:
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