Читать книгу Earth Materials - John O'Brien - Страница 56

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 shared oxygen anions to two other silica tetrahedra in the form of a long, one‐dimensional chain‐like structure. When two chains are linked through shared oxygen anions a double‐chain inosilicate structure is formed. When chains are “infinitely” linked to one another through shared oxygen anions, a two‐dimensional sheet of linked silica tetrahedra is formed which is the basic structural unit of phyllosilicates (“sheet” silicates). Finally, when silica tetrahedra are linked to adjacent silica tetrahedra by sharing all four oxygen anions, a three‐dimensional framework of linked silica tetrahedra results, which is the basic structure of tectosilicates (“framework” silicates).

Because the silicate groups constitute the most significant rock‐forming minerals in Earth's crust and upper mantle they are discussed more fully in Chapter 5. In Chapter 3, we will further investigate significant aspects of mineral chemistry, including substitution solid solution and the uses of isotopes and phase stability diagrams in understanding Earth materials.