Читать книгу Earth Materials - John O'Brien - Страница 128

Many polymorphs differ from one another only in terms of the degree of regularity in the distribution of certain ions within their respective crystal structures. Their structures can range from perfectly ordered to a random distribution of ions within structural sites (Figure 4.38). The potassium feldspar minerals (KAlSi₃O₈) provide many examples of such variation in regularity or order in the distribution of aluminum ions within the structure. In the feldspar structure, one in every four tetrahedral sites is occupied by aluminum (Al⁺³), whereas the other three are occupied by silicon (Si⁺⁴). In the potassium feldspar high sanidine, the distribution of aluminum cations is completely random (high disorder); the probability of finding an aluminum cation in any one of the four sites is equal. Crystal structures with such random distributions of cations are highly disordered and are favored by high temperatures and low pressures of formation. On the other hand, in the potassium feldspar low microcline, the distribution of aluminum cations is highly ordered (low disorder), with the aluminum distributed regularly in every fourth tetrahedral site. The probability of finding an aluminum cation in these sites approaches 100%, and the probability of finding one in the other three sites approaches zero. Crystal structures with such regular distributions of cations possess very low disorder, and their formation is favored by low temperatures and/or high pressures of formation. Intermediate degrees of order exist within the potassium feldspar group. High sanidine, with its high degree of disorder, crystallizes in the monoclinic system, and is common in volcanic rocks formed at high temperatures and low near surface pressures, whereas low microcline, with its low degree of disorder, crystallizes in the triclinic system, and is common in rocks formed at higher pressures, and in some cases lower temperatures, below the surface.

Figure 4.38 Variations in the order of minerals.

Source: Klein and Hurlbut (1985). © John Wiley & Sons.