7/30/2023 0 Comments Plot ternary diagram of feldsparsWe call any feldspar with composition near NaAlSi 3O 8, albite, and one with composition near CaAl 2Si 2O 8, anorthite, even if other components are present. Plagioclase, often called plagioclase feldspar, is mostly a solid solution of albite and anorthite, although it may contain up to 10 wt % orthoclase. Alkali feldspar, mainly solutions of orthoclase and albite, sometimes contains up to 15 wt % anorthite. Natural feldspars form two distinct series, the alkali feldspar series and plagioclase, both labeled on this triangular diagram. In Figure 6.34, the blue region shows the compositions of naturally occurring feldspars. The important feldspar end members are albite (NaAlSi 3O 8), anorthite (CaAl 2Si 2O 8), and orthoclase (KAlSi 3O 8). For most purposes, we consider them to be ternary solutions, which means we can describe their composition in terms of three end members and plot them on diagrams such as the one shown in Figure 6.34 and in Figure 2.22 (Chapter 2). Rarely, they contain significant amounts of other elements such as Ba, Sr, B, or Fe. Feldspars are solid-solution minerals and have the general formula (Ca,Na,K)(Si,Al) 4O 8. They are widespread and are essential minerals in many igneous, metamorphic, and sedimentary rocks. \)įeldspars are the most abundant minerals in Earth’s crust, in part because they contain six of the seven most abundant elements in the crust.
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