Na 0.3K 0.7AlSi 3O 8) were cooled very slowly, it would 'unmix' into Na-rich and K-rich phases - a process known as exsolution (meaning 'from solution'). At lower temperatures the alkali feldspar solid solution is not complete, and if an alkali feldspar crystal of an intermediate composition (e.g. Feldspars with compositions within this range are called alkali feldspars. The three most important feldspar minerals are orthoclase (KAlSi 3O 8), albite (NaAlSi 3O 8) and anorthite (CaAl 2Si 2O 8).Īt high temperatures there is complete solid solution between orthoclase and albite, as K and Na substitute for each other (in the same way that Fe and Mg substitute for each other in olivine). The name 'feldspar' refers to a group of silicate minerals that share the same basic structure. They crystallise from a wide spectrum of magmas and are found in many metamorphic and sedimentary rocks. Feldspars are especially common minerals and make up about 60% of the Earth's crust.
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