Feldspar element replacement

Since the primary minerals are electrically neutral a compensation must occur when differently charged elements replace each other. In plagioclase feldspars Si4+ can be replaced by Al3+ but, simultaneously Na+ is replaced by Ca2+. Large, well formed crystals are prized as gems and their colours depend on these ionic substitutions. Corundum (A1203) is colourless, yet just a few Cr3+ions turn it into a ruby (Burns, 1983). For a fuller discussion of igneous rocks the reader is referred to Dercourt and Paquet (1985). 

Indium (0.24 ppm) is similar in abundance to Sb and Cd, whereas T1 (0.7 ppm) is close to Tm and somewhat less abundant than Mo, W and Tb (1.2 ppm). Both elements are chalcophiles (p. 648), indium tending to associate with the similarly sized Zn in its sulfide minerals whilst the larger T1 tends to replace Pb in galena, PbS. Thallium(I) has a similar radius to Rb and so also concentrates with this element in the late magmatic potassium minerals such as feldspars and micas. 

This dispersed element occurs with lead in galena, PbS, where T1+ and Bp+ (or Sb +j replace two Pb +. T1+ replaces the similarly sized K+ and Rb+ (Table 2) occurring in some feldspars and micas. Thallium usually appears in the 1+ oxidation state, but appears as 3+ in the rare mineral Avicennite, TI2O3, found in Central Asia. If Tl(ni) were to appear in the ocean, it would be present as Tl(OH)3(aq). Contrary to the usual principle of hard and soft acids and bases see Hard Soft Acids and Bases), Tl(III) is softer than T1(I).21... 

Another form of nonstoichiometry arises from the partial replacement of one element by another in a crystal. It is common in natural minerals, such as the aluminosilicate feldspars (Na,Ca)(Al,Si) 408. The notation (Na,Ca) means that Na and Ca can be present in the same crystal sites in varying... 

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