Mineral phase
IMA name
Anorthite
Secondary names
Indianite
Class
silicate mineral
Compound type
tektosilicate
Comments
Anorthite: name by extension in the range Na=0-10%, K=0-10% of (Ca,Na,K)Al2Si2O8
Formula
CaAl2Si2O8
Chemical formula
CaAl2Si2O8
Elemental formula
Ca Al2 Si2 O8
Isotope mixture type
terrestrial abundance
  Formula Name Family Number min-max Isotope mole fraction State Relevance
Ca Calcium element 1 constituent element main
Al Aluminium element 2 constituent element main
Si Silicon element 2 constituent element main
O Oxygen element 8 constituent element main
Composition comments
Typical formula: Na0.05Ca0.95Al1.95Si2.05O8 - Common replacements: Na, K (limited solid solution) - Common impurities: Ti, Fe
Oxides
Formula CaO Na2O K2O Al2O3 SiO2
Mass fraction (%) 19.76 35.84 44.4
Oxides comments
frequently also Na2O ~0.6% (CaO=19.2%)

Strunz

Class
silicates (09)
Division
tektosilicate without zeolitic H2O (09.F)
Family
without additional non-tetrahedral anions, (Al,B):Si = 1:1 (09.FA)
Code
09.FA.35

Dana

Major class
silicates (VIII)
Class
tectosilicate Al-Si framework (76)
Type
with Al-Si frameworks (76.01)
Group
Plagioclase series (76.01.03)
Code
76.01.03.06
Phase type
crystalline
Crystal system
triclinic
Crystal class
pinacoidal
Crystal class symbol
-1 (Ci)
Crystal spacegroup
P-1
Molar mass
277.41 g/mol
Density
2.72 - 2.75 g/cm3
State NTP
solid
Refringence type
biaxial
Refringence sign
negative
Birefringence
0.0110 - 0.0120
Refraction index nα
1.572 - 1.576
Refraction index nβ
1.5785 - 1.5832
Refraction index nγ
1.583 - 1.588
Dispersion V
relatively weak
Pure color
colorless
Color
colorless, gray, white, red, reddish gray
Diaphaneity
transparent to translucent
Luster
vitreous