Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence its name from Latin, albus. It is a common constituent in felsic rocks.
Albite from Crete
Albite
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)