Mineral Information and Data:

(Na,Ca)(Si,Al)4O8 [CNMNC approved formula]

Named in 1841 after locality; in Andes Mountains, South America, where it is abundant in the lavas.

IMA status : Intermediate Species Member; 1841 ()
(8th edition) :  
  (76) Tectosilicate Al-Si Framework
  (76.01) with Al-Si frameworks
  (76.01.03) Plagioclase series
(10th edition) :
  (09) Silicates
  (09.F) Tektosilicates without Zeolitic H2O
  (09.FA) Tektosilicates without additional non-tetrahedral anions
Crystal system: Triclinic System
Point group (H-M): 1 — pinacoidal
Unit cell: a = 8.15 Å, b = 12.83 Å, c = 14.21 Å, a = 93.6°, b = 116.2°, g = 89.7°
Crystal Habit: Crystals rare, commonly massive, cleavable or granular
Twinning: Common by Albite, Carlsbad, or Pericline law, also similar to Manebach law
Color: White, gray, green, yellow, orange-red to flesh-red
Diaphaneity: Transparent to translucent
Luster: Subvitreous to pearly
Hardness (Mohs): 6 to 6.5
Measured Density: 2.66 to 2.68 g/cm3
Cleavage: 3; {001} perfect, {010} imperfect, {110} marginal
Tenacity: Brittle
Fracture: Uneven to conchoidal
Streak: White
Polymorphism and Series: Low and high temperature variants differing structural detail are recognized
Geologic Setting: 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
Mineral Association: Quartz, potassic feldspar, biotite, hornblende series, magnetite
Synonyms/varieties: Plagioclase [An 30-50]
Other Properties: Gemstone Unlikely Available.
Comments: Intermediate member of plagioclase series, feldspar group.


Andesine on matrix
Origin: Boulouris, St. Raphael, France
Sample size: thumbnail
Photo: Diederik Visser