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)
Anorthite
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Anorthite is the calcium endmember of the plagioclase feldspar mineral series. The chemical formula of pure anorthite is CaAl2Si2O8. Anorthite is found in igneous rocks.
Plagioclase
…feldspar series. This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi3O8 to CaAl2Si2O8), where sodium and calcium atoms can substitute for each other in the mineral's crystal…
Anorthite crystals in a basalt vug from Vesuvius (size:6.9 × 4.1 × 3.8 cm)
Anorthite crystals (white) in lava from Miyake Island, Japan (size: 2.4 × 1.7 × 1.7 cm)