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Plagioclase is found in almost all igneous rocks and most metamorphic rocks, but is less common in sedimentary rocks where it usually weathers to clay minerals or a fine grained variant of muscovite (sericite). Chemical composition - CaAl 2 Si2O 8 (anorthite), NaAlSi 3 O 8 (albite) Hardness - 6-6.5 Specific gravity - 2.6-2.8
Any of a series of common feldspar minerals, consisting of differing mixtures of sodium and calcium aluminum silicates. Plagioclase is typically white, yellow, or reddish-gray, but it can also be blue to black. It occurs in igneous rocks. The minerals albite, oligoclase, andesine, labradorite, bytownite, and anorthite are all plagioclases.
Composed of essential calcic plagioclase and pyroxene, sometimes with olivine, feldspathoids or interstitial quartz. Andesite: an intermediate, commonly porphyritic volcanic rock composed of plagioclase, pyroxene, hornblende and/or biotite. Latite: an intermediate volcanic rock with equal amounts of sodic plagioclase and alkali feldspar.
Anorthosite / æ n ˈ ɔːr θ ə s aɪ t / is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present.. Anorthosites are of enormous geologic interest, because it is still not fully understood how they form.
Feldspar - Feldspar - Identification of specific feldspars: Alkali feldspars can often be distinguished from plagioclase feldspars because most grains of the latter exhibit albite twinning (see above Crystal structure), which is manifested by parallel lines on certain cleavage surfaces, whereas grains of alkali feldspars do not. This criterion is not, however, absolute; some plagioclase ...