Plagioclase
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Plagioclase is a series of tectosilicate minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solut
A photomicrograph of a plagioclase crystal (gray) under cross polarized light. The plagioclase crystal shows a distinct banding effect called polysynthetic twinning.
Plagioclase displaying cleavage (unknown scale)
In volcanic rocks, fine-grained plagioclase can display a "microlitic" texture of many small crystals.
Labradorite displaying typical iridescent effect termed labradorescence (unknown scale)
Granite
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Granite is a coarse-grained intrusive igneous rock composed mostly of quartz, alkali feldspar, mica and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slow
Plagioclase
…characteristic mineral of rocks such as diorite which contain a moderate amount of silica and related volcanics such as andesite. Oligoclase is common in granite and monzonite. The name oligoclase is derived from the Greek olígos ('small, slight') + klásis ('fracture'), in reference to the fact that its cleavage…
Granite containing plagioclase feldspar, potassium feldspar, quartz, biotite, and/or amphibole
Granite with a moderate amount of plagioclase, alkali feldspar, and quartz
Granite with a composition of alkali feldspar and quartz
Granite in thin section, under cross-polarized light