The Silicate Tetrahedron

The silicate anion is made of four large oxygen atoms and one small silicon atom that pack themselves together to occupy the smallest possible space. This shape, with big oxygen atoms at four corners of the structure and the silicon atom at the center, is known as the silicate tetrahedron. Each silicate tetrahedron has four unsatisfied negative charges (Si has a charge of +4, whereas each oxygen has a charge of -2). To make a stable compound the silicate tetrahedron must therefore combine to neutralize this extra charge, which can happen in one of two ways:

(1) Oxygen can form bonds with cations (positively charged ions). For instance, Mg2+ has a charge of +2, and by combining with

Mg2+, the silicate tetrahedron makes a mineral called olivine (mg2)sio4. (2) Two adjacent tetrahedra can share an oxygen atom, making a complex anion with the formula (si2o7)6-. This process commonly forms long chains, so that the charge is balanced except at the ends of the structure. This process of linking silicate tetrahedra into large anion groups is called polymerization, and it is the most common way to build minerals, but in making the various possible combinations of tetrahedra, one rule must be followed, that is tet-rahedra can only be linked at their apexes.

olivine is one of the most important minerals on Earth, forming much of the oceanic crust and upper mantle. It has the formula (mg,Fe)2sio4 and forms the gem peridot.

Garnet is made of isolated silicate tetrahedra packed together without polymerizing with other tetrahedra. There are many different kinds of garnets, with almandine being one of the more common, deep red varieties that forms a common gemstone. Ionic substitution is common, with garnet having the chemical formula A3B2(sio4) 3, where:

Silicon ion (Si4+)

Silicon ion (Si4+)

Silicate TetrahedraFlores Tela Azul Dibujos

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Pyroxene and amphibole both contain continuous chains Diagram showing silicate tetrahedra of silicate tetrahedra. Polymerized chains of tetrahedra form pyroxenes, whereas amphiboles are built in double chains or linked rings. In both of these structures, the chains are bound together by cations such as Ca2+, Mg2+, and Fe2+, which satisfy the negative charges of the polymerized tetrahedra. Pyroxenes are very common minerals in the oceanic crust and mantle and also occur in the continental crust. Amphiboles are very common in metamorphic rocks, have a complicated the sheets. The sheet structure is why micas are easy to peel apart on cellophane-like surfaces.

Quartz, one of the most common minerals, also has one of the most common polymerizations. Its charges are satisfied by sharing all of its oxygen in a three-dimensional network. Quartz typically has six-sided crystals and has many other different forms and colors.

Feldspars are the most common minerals in the Earth's crust. They account for 60 percent of all minerals in the continental crust and 75 percent of the volume. Feldspars are also common in the oceanic crust. Like quartz, feldspars have a structure formed by polymerization of all the oxygen atoms, and some of the silicon atoms are replaced by Al3+. Many different kinds of feldspar minerals form by the addition of different cations to the structure. For instance, potassium feldspar has the formula K(si3Al)o8, albite has the formula Na(si3Al)o8, and anorthite has the formula Ca(si2Al2)o8. A complete range of chemical compositions of feldspars is possible between the albite and anorthite varieties. These feldspar minerals are known as the plagioclase feldspars.

silicates are the most abundant rock-forming minerals, but other types do occur in sufficient quantities to call them rock-forming minerals. oxides use the oxygen anion and include ore minerals such as chromium, uranium, tin, and magnetite (Feo4). sulfides are minerals such as pyrite, copper, lead, zinc, cobalt, mercury, or silver that combine with the sulfur anion. For instance, Fes2 is the formula for pyrite, commonly known as fool's gold. The carbonates calcite, aragonite, and dolomite form with the complex carbonate anion (Co3)2-. Phosphates are formed using the complex anion (Po4)3-. An example is the mineral apatite, used for fertilizers, and the same substance as that which forms teeth and bones. sulfate minerals are formed using the complex sulfate

Uncut emerald from Colombia (Carl Frank/Photo Researchers, Inc.)

ion (so4)2-. Gypsum and anhydrite, sulfate minerals formed by evaporation of salt water, are commonly used to make plaster.

one of the most important nonsilicate mineral groups is the carbonates, which are built using the carbonate ion. The most common carbonates are calcite (CaCo3) and dolomite ((Ca,mgCo3)2). These minerals are common in sedimentary rock sequences deposited under marine conditions. other nonsilicate minerals common in sedimentary rocks include the halide mineral halite (NaCl) and the sulfate gypsum (Caso4), both common in evaporate sequences formed when ocean waters evaporate and leave the dissolved minerals behind as sedimentary layers.

Native elements are not as common as many other mineral groups, and consist of free-occurring elements such as gold (Au), copper (Cu), silver (Ag), platinum (Pt), diamond (C), and graphite (C). Non-native-element economically important minerals include some oxides (hematite, magnetite, chromite, and ilmenite) and sulfides such as pyrite, chalcopy-rite, galena, and sphalerite.

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