Constituent
- Name
- Anorthite
- IMA name
- Anorthite
- Class
- silicate mineral
- Compound type
- tektosilicate
- Comments
-
generaly crystalline
Anorthite: name by extension in the range Na=0-10%, K=0-10% of $(Ca,Na,K)Al_2Si_2O_8$
- Relevance
- main - major
- Arrangement
- single phase
- Mass fraction
- 1.0 $\pm$ 0.001
- Formula
- $(Ca_{0.73},Na_{0.21},K_{0.06})(Al_{0.79},Si_{0.21})AlSi_2O_8$
- Chemical formula
- CaAl2Si2O8
- Elemental formula
- Ca0.73 Na0.21 K0.06 Al1.79 Si2.21 O8
- Isotope mixture type
- terrestrial abundance
- Composition comments
- Typical formula: Na0.05Ca0.95Al1.95Si2.05O8 - Common replacements: Na, K (limited solid solution) - Common impurities: Ti, Fe - from "Electron microprobe analysis shows that Plagioclase is Anorthite ($Ca_{73}Na_{21}K_6$)" = Bytownite; Allen et al. 1998
- Oxides
-
Formula $CaO$ $Na_{2}O$ $K_{2}O$ $Al_{2}O_{3}$ $SiO_{2}$ Mass fraction (%) 14.9 2.4 1.1 33.2 48.4 - Oxides comments
- frequently also Na2O ~0.6% (CaO=19.2%) - calculated with elemental formula
Strunz
- Class
- silicates (09)
- Division
- tektosilicate without zeolitic H2O (09.F)
- Family
- Tektosilicates 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
- Texture
- polycrystalline
- Molar mass
- 274.8 $g/mol$
- Density
- 2.70 $g/cm^3$
- Refringence type
- biaxial
- Refringence sign
- negative
- Birefringence
- 0.0110 - 0.0120
- Refraction index $n_{\alpha}$
- 1.572 - 1.576
- Refraction index $n_{\beta}$
- 1.5785 - 1.5832
- Refraction index $n_{\gamma}$
- 1.583 - 1.588
- Dispersion V
- relatively weak
- Color
- colorless, gray, white, red, reddish gray
- Diaphaneity
- transparent to translucent
- Luster
- vitreous