Earth Materials. John O'Brien

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Figure 2.20 A silica tetrahedron is formed when four oxygen ions (O⁻²) bond to one silicon ion (Si⁺⁴) in the form of a tetrahedron. The electrostatic valency of each silicon–oxygen bond in the silica tetrahedron is one charge unit, which fully neutralizes the charge on the central silicon ion (four = four), while leaving the charge on the oxygen ions only partially neutralized (one is one‐half of two).

Table 2.7 Mineral classification based on the major anion groups.

      Oxygen (O) and silicon (Si) are the two most abundant elements in Earth's continental crust, oceanic crust and mantle. Under the relatively low pressure conditions that exist in the crust and the upper mantle, the most abundant rock‐forming minerals (Chapter 5) are silicate minerals. Silicate minerals, characterized by the presence of silicon and oxygen that have bonded together to form silica tetrahedra, are utilized here to show how coordination polyhedra are linked to produce larger structures with the potential for the long‐range order characteristic of all minerals.

      2.5.1 The basics: silica tetrahedral linkage

Silica tetrahedra are composed of a single, small, tetravalent silicon ion (Si⁺⁴) in fourfold, tetrahedral coordination with four larger, divalent oxygen ions (O⁻²).These silica tetrahedra may be thought of as the basic building blocks, the LEGO®, of silicate minerals. Because the electrostatic valency of each of the four Si–O bonds in the tetrahedron is one (EV = 1), the +4 charge of the silicon ion is effectively neutralized. However, the −2 charges on the oxygen (O⁻²) ions are not neutralized. Each oxygen ion possesses an unsatisfied charge of −1 which it can only neutralize by bonding with one or more additional cations in the mineral structure. Essentially, as a crystal forms, oxygen anions can bond to another silicon (Si⁺⁴) ion to form a second bond with an electrostatic valency of 1 or it can bond to some other combination of cations (e.g., Al⁺³, Mg⁺², Fe⁺², Ca⁺², K⁺¹, Na⁺¹) with a total electrostatic valency of 1.

      Many factors influence the type of silica tetrahedral structure that develops when silicate minerals form; the most important is the relative availability of silicon and other cations in the environment in which the mineral crystallizes. Environments with abundant silicon (and therefore silica tetrahedra) tend to favor the linkage of silica tetrahedra through shared oxygen ions. Environments depleted in silicon tend to favor the linkage of the oxygen ions in silica tetrahedra to cations other than silicon. In such situations, silica tetrahedra tend to link to coordination polyhedral elements other than silica tetrahedra.

      If none of the oxygen ions in a silica tetrahedron bond to other silicon ions in adjacent tetrahedra, the silica tetrahedron will occur as an isolated tetrahedral unit in the mineral structure. If all the oxygen ions in a silica tetrahedron bond to other silicon ions of adjacent tetrahedra, the silica tetrahedra form a three‐dimensional framework structure. If some of the oxygen ions in the silica tetrahedra are bonded to silicon ions in adjacent tetrahedra and others are bonded to other cations in adjacent coordination polyhedra, a structure that is intermediate between totally isolated silica tetrahedra and three‐dimensional frameworks of silica tetrahedra will develop.

Six major silicate groups (Figure 2.21) are distinguished based upon the linkage patterns of silica tetrahedra. These are: (1) nesosilicates, (2) sorosilicates, (3) cyclosilicates, (4) inosilicates, (5) phyllosilicates, and (6) tectosilicates. Nesosilicates (“island” silicates) are characterized by isolated silica tetrahedra that are not linked to other silica tetrahedra through shared oxygen ions. Sorosilicates (“bow‐tie” silicates) contain pairs of silica tetrahedra linked through shared oxygen ions. In cyclosilicates (“ring” silicates), each silica tetrahedron is linked to two other tetrahedra through shared oxygen ions into ring‐shaped structural units. In single‐chain inosilicates, each silica tetrahedron is linked through СКАЧАТЬ