Anorthosite 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.
Anorthosite from Salem district, Tamil Nadu, India
Nain Anorthosite, a mid-Mesoproterozoic intrusion (1.29 to 1.35 billion years), Labrador. Polished slab; blue color is labradorescence.
Anorthosite from southern Finland
Anorthosite from Poland
Plagioclase ( PLAJ-(ee)-ə-klayss, PLAYJ-, -klayz) is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more properly known as the 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 lattice structure. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or "record-groove" effect.
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)