Class

silicate mineral

Compound type

tektosilicate

Comments

generaly crystalline
Anorthite: name by extension in the range Na=0-10%, K=0-10% of $(Ca,Na,K)Al_2Si_2O_8$

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

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