In all feldspars, Na, K, or Ca occupy spaces between tetrahedra. The purple atoms are sodium, and the yellow and green tetrahedra are (SiO 4) and (AlO 4), respectively. As an example, Figure 6.36 shows the atomic arrangement in albite. Unlike quartz, feldspars contain both SiO 4 and AlO 4 tetrahedra. Like quartz, feldspars are framework silicates. Figure 6.36: The arrangement of atoms in feldspar Thus, the Grorud feldspar has composition An 3Ab 62Or 35. Often, we describe the compositions of feldspars by using abbreviations with subscripts. For this feldspar, X An = 3, X Ab = 62, and X Or = 35, where the symbol X stands for mole fraction. It has a composition of about (Ca 0.03Na 0.62K 0.35) (Al 1.03Si 2.97)O 8, so it contains 3 mol % anorthite (CaAl 2Si 2O 8), 62 mol % albite (NaAlSi 3O 8), and 35 mol % orthoclase (KAlSi 3O 8). This feldspar is an example of anorthoclase. The triangular diagram in Figure 6.35 is similar to the one in Chapter 2 (section 2.5.1) it shows the composition of a feldspar from Grorud, near Oslo, Norway. Figure 6.35: The composition of the Grorud feldspar The term anorthoclase, however, is often used to refer to alkali feldspars with intermediate compositions (10-36% KAlSi 3O 8 with the rest being mostly NaAlSi 3O 8). The alkali feldspars, with compositions between NaAlSi 3O 8 and KAlSi 3O 8 have not been divided into as many bins as plagioclase feldspars. It is also the name we give to any feldspar that is >90 KAlSi 3O 8. Orthoclase, for example, is the name given to end member KAlSi 3O 8. Compositions between plagioclase and alkali feldspar (that would plot in the white part of the triangle) are rare or do not exist.Ĭonfusion sometimes arises because the names of some composition ranges are the same as the names of feldspar end members (albite, anorthite, orthoclase). ![]() Labradorite may also contain a small amount of orthoclase. Labradorite, for example, is plagioclase with composition between 70% anorthite-30% albite and 50% anorthite-50% albite. ![]() Intermediate plagioclase compositions are commonly given specific names (labeled in the figure): oligoclase, andesine, labradorite, and bytownite. We 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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