Feldspars (sometimes spelled felspars) are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. Ternary Feldspar Experiments and Thermodynamic Models. This causes Plagioclase to be iridescent. Compositional phase diagram of the different minerals that constitute the feldspar solid solution. We can demonstrate this in action, via attempting to plot a ternary diagram with a mapping. 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. Exsolution patterns are subtle in plagioclase and occur because of incompatibilities in the Al/Si ordering modes (Nesse 2012). The possible forms of Plagioclase are listed below.Įxsolution occurs when the mineral cools after crystallization. Plagioclase nomenclature is based on the percent of anorthite in the sample. The solid solution equilibrium equation can be written as Na+1 Si+4 Ca+2 Al+3 (Nesse 2012). Composition of Plagioclase is described in terms of Anorthite with An + Ab = 100. In plutonic rocks, all of the minerals are crystallized into visible grains. Ca2+ can substitute for Na+ because they are similar in size and charge neutrality is maintained by substituting Al3+ for Si4+. The QAP ternary diagram is used to classify igneous rocks with visible mineral grains (phaneritic texture) from their feldspar and quartz content. Albite is at one end of the series and Anorthite at the other.
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