Anorthite

Formula:  (Ca_x Na_1-x)(Si_3-xAl_1+x)O₈  (x = 0.9 - 1.0)

Origin of name:  From Greek an = not and orthos = right (angle), for it triclinicity

Crystal system:  Triclinic

Crystal class:  -1

Twinning:  Common by albite law; also by Carlsbad, Baveno, Manebach, or Pericline law

Unit cell:   a = 8.18 Å, b = 12.88 Å, c = 14.17 Å, a = 93.2°, b = 115.9°, g = 91.2°

Colour:  Colourless, white, greyish, reddish

Diaphaneity: Transparent to translucent

Luster:  Vitreous

Habit:  Crystals usually short prismatic; massive, cleavable

Hardness:  6 to 6.5

Specific gravity:  2.74 to 2.76

Cleavage:  3; {001} perfect, {010} nearly perfect, {110} imperfect

Tenacity:  Brittle

Streak:  White

Synonyms/varieties:  Plagioclase [An 90-100]

Comments:  End member of the plagioclase series, feldspar group. A rare constituent of mafic plutonic and volcanic rocks. In some granulite facies metamorphic rocks; in metamorphosed carbonate rocks; with corundum deposits. Known from meteorites.