Andesine

Formula:  (Ca_xNa_1-x)(Si_3-xAl_1+x)₄O₈   (x = 0.3 to 0.5)

Origin of name:  After locality; in Andes Mountains, South America, where it is abundant in the lavas

Crystal system:  Triclinic

Crystal class:  -1

Twinning:  Common by Albite, Carlsbad, or Pericline law, also similar to Manebach law

Unit cell:   a = 8.15 Å, b = 12.83 Å, c = 14.21 Å, a = 93.6°, b = 116.2°, g = 89.7°

Colour:  White, gray, green, yellow, orange-red to flesh-red

Diaphaneity: Transparent to translucent

Luster:  Subvitreous to pearly

Habit: crystals rare, commonly massive, cleavable or granular

Hardness:  6 to 6.5

Specific gravity:  2.66 to 2.68

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

Tenacity:  Brittle

Streak:  White

Synonyms/varieties:  Plagioclase [An 30-50]

Comments:  Not IMA approved. Intermediate member of plagioclase series, feldspar group. Widespread in igneous rocks of intermediate silica content, as syenites and andesites. Characteristic of granulite to amphibolite facies metamorphism, commonly as antiperthite; as detrital grains in sedimentary rocks.