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Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or "record-groove" effect. Plagioclase is a major constituent mineral in Earth's crust and is consequently an important diagnostic tool in petrology for identifying the composition, origin and evolution of igneous rocks.
Twinning is crystallographically defined by its twin plane 𝑲 𝟏, the mirror plane in the twin and parent material, and 𝜼 𝟏, which is the twinning shear direction. Deformation twins in Zr are generally lenticular in shape, lengthening in the 𝜼 𝟏 direction and thickening along the 𝑲 𝟏 plane normal.
Bytownite is a calcium rich member of the plagioclase solid solution series of feldspar minerals with composition between anorthite and labradorite.It is usually defined as having between 70 and 90%An (formula: (Ca 0.7−0.9 Na 0.3−0.1)[Al(Al,Si)Si 2 O 8]).
Orthoclase crystal twinning from the Organ Mountains in New Mexico. Orthoclase is a common constituent of most granites and other felsic igneous rocks and often forms huge crystals and masses in pegmatite. Typically, the pure potassium endmember of orthoclase forms a solid solution with albite, the sodium endmember (NaAlSi 3 O 8), of plagioclase.
Labradorite is an intermediate to calcic member of the plagioclase series. It has an anorthite percentage (%An) of between 50 and 70. The specific gravity ranges from 2.68 to 2.72. The streak is white, like most silicates. The refractive index ranges from 1.559 to 1.573 and twinning is common.
Anorthite is a rare compositional variety of plagioclase. It occurs in mafic igneous rock. It also occurs in metamorphic rocks of granulite facies, in metamorphosed carbonate rocks, and corundum deposits. [3] Its type localities are Monte Somma and Valle di Fassa, Italy. It was first described in 1823. [5]
Plagioclase for example can be seen in the photo on the right as a clear mineral with multiple parallel twinning planes. The large blue-green minerals are clinopyroxene with some exsolution of orthopyroxene. Thin sections are prepared in order to investigate the optical properties of the minerals in the rock.
Primary minerals in I-type granites are plagioclase, potassium feldspar, and quartz as in S- and A-type granites. I-type granites have less quartz then their S-type granite color index equivalents. Plagioclase displays zonation and albite twinning. Potassium feldspar can show perthite textures, carlsbad twinning, and, in microcline, tartan ...