Plagioclases are triclinic feldspars that form a complete isomorphic compounds which are the final members of the Na–plagioclase albite NaAlSi 3 O 8 (Ab) and Ca–plagioclase anortite CaAl 2 Si 2 O 8 (An) (Table 2.13).In albite one of the four Si 4+ ions is isomorphic substituted with one Al 3+ ions, and one free (−) valence is related to Na + ion. Compositions of individual samples are usually written as An 24 Ab 76 meaning in this case that it is 24% anorthite and 76% albite. The composition of plagioclase feldspar is typically indicated by the general anorthite (% An) or albite (% Ab) fraction and is easily determined by measuring the refractive index or peel angle within the crushed particles by measuring the refraction angle under a thin section. The twinning in plagioclase produces stacks of twin layers that are typically fractions to several millimetres thick. Alternate name for the plagioclase series of feldspars. Formula: NaAlSi 3 O 8 – CaAl 2 Si 2 O 8 Triclinic Description: Plagioclase is a family of feldspar minerals forming a solid solution that ranges, relatively continuously, from pure sodium aluminum silicate (the end member called albite) to pure calcium aluminum silicate (the end member called anorthite). In petrology, the plagioclase series are defined based on their percentage of albite (Ab) to anorthite (An). Plagioclase Plagioclase is a member of the feldspar group (like orthoclase) and is a framework silicate.Plagioclase consists of a solid solution between the albite and anorthite end-members, and together with quartz is the most common of the rock forming minerals. Twinning can be seen in the image of Plagioclase below. (Nelson 2012) The general chemical formula is a bit complicated (K,Na) 1-x (Ca,Ba) x (Si 3-x Al 1+x O 8).This complexity is caused by presence of both monovalent and divalent cations which require more complicated heterovalent substitution. Alkali feldspar has the formula KAlSi 3 O 8, potassium aluminosilicate. The formula actually is a blend ranging from all sodium (albite) to all potassium (microcline), but albite is also one endpoint in the plagioclase series so we classify it there. General Plagioclase Information : Chemical Formula: (Na,Ca)(Si,Al)4O8 : Composition: Molecular Weight = 270.77 gm Sodium 4.25 % Na 5.72 % Na 2 O: Calcium 7.40 % Ca 10.36 % CaO: Aluminum 9.96 % Al 18.83 % Al 2 O 3: Silicon 31.12 % Si 66.57 % SiO 2: Oxygen 47.27 % O: 100.00 % 101.48 % = TOTAL OXIDE In thin section, Plagioclase commonly shows the characteristic albite polysynthetic twinning. This twinning is the most characteristic identifying feature of Plagioclase, and makes its identification easy when present. The chemical composition of feldspars includes alcalic metals, aluminum, silica and oxygen.
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