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. Alternate name for the plagioclase series of feldspars. In petrology, the plagioclase series are defined based on their percentage of albite (Ab) to anorthite (An). Chemical Formula: (Na,Ca)(Si,Al)4O8 : Composition: Molecular Weight = 265.42 gm Sodium 6.93 % Na 9.34 % Na 2 O Calcium ... Oligoclase Plagioclase. This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. The key difference between orthoclase and plagioclase is that orthoclase appears in green-yellow colour, whereas plagioclase appears in white. Plagioclase is a mixture of albite (Ab), or sodium aluminosilicate (NaAlSi 3 O 8), and anorthite (An), or calcium aluminosilicate (CaAl 2 Si 2 O 8); the two intermingle and form a continuous chemical range (called a solid-solution series) between albite and anorthite (the end-members of the series). The other end member, Anorthite, contains calcium and no sodium. Plagioclase is a continuous series of solid solutions known as the plagioclase feldspar series, rather than a specific mineral with a particular chemical composition. Comments: Clear glassy crystalline oligoclase with milky quartz and muscovite. Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Albite is one end member, containing sodium and no calcium. The twinning in plagioclase produces stacks of twin layers that are typically fractions to several millimetres thick. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more properly known as the plagioclase feldspar series. 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. Anorthite belongs to the Plagioclase Feldspar group, an isomorphous solid solution series. Plagioclase is an important mineral in the feldspar group, and the chemical formula of this mineral is NaAlSi 3 O 8 – CaAl 2 Si 2 O 8.