Igneous mineral, stability

The mineral structures, of igneous minerals are varying organizations of silicon-oxygen tetrahedra and aluminium-oxygen tetrahedra and octahedra. The stability of igneous minerals also applies to the weathering of chemically and structurally similar minerals in metamorphic mid sedimentary rocks. 

Figure 7.5 is approximate because the AG values of formation of Ca- and Na-montmorillonites are educated guesses. Figure 7.5 also ignores the slow kinetics of silicate reactions. In addition, die activities of Mg2+, K+, andNa+ affect the stability of other soil minerals and their igneous minerals, which direcdy and indirecdy affect the stability of Ca minerals. 

Frost, B.R. Lindsley, D.H. (1991) Occurrence of iron-titanium oxides in igneous rocks. In Lindsley, D.H. (ed.) Oxide minerals. Reviews in Mineralogy 25, Min. Soc. Am., 433-468 Frost, B.R. (1991) Stability of oxide minerals in metamorphic rocks. In Lindsley, D.H. (ed.) Oxide minerals. Reviews in Mineralogy 25, Min. Soc. Am., 469-488 Fryer, J.R. (1979) The chemical applications of transmission electron microscopy. Academic Press, London New York, 231 p. 

Gooding, J. L. (1978) Chemical weathering on Mars thermodynamic stabilities of primary minerals (and their alteration products) from mafic igneous rocks. 

FIGURE 7.2. Stability to weathering of some minerals in igneous and metamorphic rocks. (Adapted from S. S. Goldich. 1938. J. Geol. 46 38.)... 

The K and Na stability diagrams are similar, but the values of M/H vary. The Ca feldspars are the most unstable, and the K feldspars are the most stable, with respect to weathering. At higher M/H ratios (more basic solutions) other minerals are stable and would precipitate before the igneous feldspars, but free energy data for other silicates are lacking. 

The composition of a sedimentary rock depends partiy on the composition of the parent material and the stability of its component minerals, and partly on the type of action to which the parent rock was subjected and the length of time it had to suffer such action. The least stable minerals tend to be those that are developed in environments very different from those experienced at the Earth s surface. In fact, quartz, and, to a much lesser extent, mica, are the only common detrital constituents of igneous and metamorphic rocks that are found in abundance in sediments. Most of the other minerals are ultimately broken down chemically to give rise to clay minerals. The more mature a sedimentary rock is, the more it approaches a stable end product, and very mature sediments are likely to have experienced more than one cycle of sedimentatbn. 

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