San Bernardino Volcanic Field, Cochise Co., Arizona, USA

(Ca,Na)(Si,Al)₄O₈

Crystal System: triclinic
Colour: white to grey, bluish, greenish, reddish/ colourless, reddish grey, white
Lustre: vitreous
Habitus: pseudo-prismatic, tabular
Hardness: 6-61/2
Fracture: irregular/uneven, concohidal
Cleavage: perfect on {001}, good on {010} , imperfect on {110} / Perfect {001}, less so on {010}, imperfect on {110}
Density: 2.615 g/cm³
Origin and geological occurrence: plagioclase means a solid solution between anorthite (CaAl₂Si₂O₈) and albite (NaAlSi₃O₈). It is a widespread mineral in many environments from magmatic to metamorphic. The femic rocks show higher calcium content.

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Samples:

9F.20 Plagioclase Euhedral prismatic plagioclase phenocrysts in basalt #tectosilicates