Earth Materials. John O'Brien. Читать онлайн. Newlib. NEWLIB.NET

Автор: John O'Brien
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: География
Год издания: 0
isbn: 9781119512219
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2.20). The electrostatic valency of each bond is given by EV = Z/CN = +4/4 = +1. What this means is that each bond between the coordinating silicon ion (Si+4) and the coordinated oxygen ions (O−2) balances a charge of +1. Another way to look at this is to say that each bond involves an electrostatic attraction between ions of opposite charge of one charge unit. Since there are four Si–O bonds, each balancing a charge of +1, the +4 charge on the silicon ion is fully neutralized by the four nearest neighbor anions to which it is bonded. However, although the +4 charge on the coordinating silicon ion is fully satisfied, the −2 charge on each of the coordinated ions is not. Since each has a −2 charge, a single bond involving an electrostatic attraction of one charge unit neutralizes only half their charge. They must attract and bond to one or more additional cations, with an additional total electrostatic valency of one, in order to have their charges effectively neutralized. So it is that during mineral growth, cations attract anions and anions attract additional cations of the appropriate charge and radius which in turn attract additional anions of the appropriate charge and radius as the mineral grows. In this manner minerals retain their essential geometric patterns and their ions are neutralized as the mineral grows. In the following section we will introduce the major mineral groups and see how their crystal chemistry forms the basis of the mineral classification.

Schematic illustration of common coordination polyhedra: (a) cubic closest packing, (b) cubic, (c) octahedral, (d) tetrahedral, (e) triangular, (f) linear.

      Source: Wenk and Bulakh (2004). © Cambridge University Press.

Ion CN = 4 CN = 6 CN = 8
Na+1 0.99 1.02 1.18
K+1 1.38 1.51
Rb+1 1.52 1.61
Cs+1 1.67 1.74
Mg+2 0.57 0.72
Al+3 0.39 0.48
Si+4 0.26 0.40
P+5 0.17 0.38
S+6 0.12 0.29

Schematic illustration of a silica tetrahedron is formed when four oxygen ions (O-2) bond to one silicon ion (Si plus 4) in the form of a tetrahedron.

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Mineral group Major anion groups Mineral group Major anion groups
Native elements None Nitrates (NO3)−1
Halides F−1, Cl−1, Br−1 Borates (BO3)−3 and (BO4)−5
Sulfides S−2, S−4 Sulfates (SO4)−2
Arsenides As−2, As−3 Phosphates (PO4)−3
Sulfarsendies As−2 or As−3 and S−2 or S−4 Chromates (CrO4)−5
Selenides Se−2 Arsenates (AsO4)−3
Tellurides Te−2 Vanadates (VO4)−3
Oxides O−2