This causes Plagioclase to be iridescent. analysis R.BLANK f Berchtesgadensaltclay(G MBELcitedby SCHAUBERGER. Exsolution usually cannot be seen in hand sample or microscopically but the thickness of repeating lamellae is about the same as the wavelength of visible light and can serve as a diffraction grating. means of a ternary diagram withK2O,Al 2O3 and MgO(in weight percent) (Fig. Figure 5.28 Ternary diagram showing the feldspar group of framework silicate minerals. The distance along a side of the diagram represents the relative abundance of the composition of each end-member. Thus, to show graphically the variation in chemical composition of the feldspars, a ternary composition diagram is needed with two independent composition axes and three end member components: Or, Ab, and An (see at- tached diagram). Exsolution patterns are subtle in plagioclase and occur because of incompatibilities in the Al/Si ordering modes (Nesse 2012). This system is illustrated with a triangular diagram that has each end-member at one corner (Figure 5.28). The possible forms of Plagioclase are listed below.Įxsolution occurs when the mineral cools after crystallization. Feldspar minerals are the most abundant constituents of igneous rocks. This can be illustrated in the feldspar compositional ternary diagram. Minerals of the feldspar group consist of three compositional end members. Plagioclase nomenclature is based on the percent of anorthite in the sample. The Feldspar Compositional Ternary Diagram NovemChristian S. Depending on temperature and other environmental factors, alkali (Na K) feldspars contain < 5 10 mol An, whereas. The results may be easily plotted on the feldspar ternary diagram (left). The solid solution equilibrium equation can be written as Na+1 Si+4 Ca+2 Al+3 (Nesse 2012). Feldspar compositions are often expressed in terms of their molecular percentages of anorthite (An), albite (Ab), and orthoclase (Or). Composition of Plagioclase is described in terms of Anorthite with An + Ab = 100. Ca2+ can substitute for Na+ because they are similar in size and charge neutrality is maintained by substituting Al3+ for Si4+. Albite is at one end of the series and Anorthite at the other.

Feldspar ternary diagram blank