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. Intersection of the lines gives compositions C 1 and C 2 as shown.įraction of solid β = (65 - 58) / (92 - 58) = 20 weight%įraction of liquid = (92 - 65) / (92 - 58) = 80 weight%įraction of solid β = (65 - 48) / (87 - 48) = 44 weight%.Īs the alloy is cooled, more solid β phase forms.Īt point 4, the remainder of the liquid becomes a eutectic phase of α+ β andįraction of eutectic = 56 weight% Point 5įraction of solid β = (65 - 9) / (91 - 9) = 68 weight%įraction of solid α = (91 - 65) / (91 - 9) = 32 weight%.\)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. A tie-line is a horizontal (i.e., constant-temperature) line through the chosen point, which intersects the phase boundary lines on either side.Ī tie-line is drawn through the point, and the lever rule is applied to identify the proportions of phases present. The green dashed line below is an example of a tie-line. ![]() Phase β first forms with a composition of 96 weight% B. The proportions of the phases present are determined by the weights needed to balance the system.Īt point 1 the alloy is completely liquid, with a composition C.Īt point 2 the alloy has cooled as far as the liquidus, and solid phase β starts to form. The composition of the alloy is represented by the fulcrum, and the compositions of the two phases by the ends of a bar. The lever rule can be explained by considering a simple balance. If an alloy consists of more than one phase, the amount of each phase present can be found by applying the lever rule to the phase diagram.
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