Secondary names

Plagioclase feldspar

Composition comments

Common replacement: K (limited solid solution) - Common impurities: Ti, Fe, Mg, H2O

Oxides

Formula	$Na_{2}O$	$CaO$	$K_{2}O$	$Al_{2}O_{3}$	$SiO_{2}$
Mass fraction (%)

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)

End-members

• Albite
• Anorthite

Members

• Albite (Na (Ab) = 90-100% - Ca = 0-10% - K (Or) = 0-10% (triclinic))
• Oligoclase (Na (Ab) = 70-90% - Ca = 10-30% - K (Or) = 0-10% (triclinic))
• Andesine (Na (Ab) = 50-70% - Ca = 30-50% - K (Or) = 0-10% (triclinic))
• Labradorite (Na (Ab) = 30-50% - Ca = 50-70% - K (Or) = 0-10% (triclinic))
• Bytownite (Na (Ab) = 10-30% - Ca = 80-90% - K (Or) = 0-10% (triclinic))
• Anorthite (Na (Ab) = 0-10% - Ca (An) = 90-100% - K (Or) = 0-10% (triclinic))

Phase type

crystalline

Crystal system

triclinic

Crystal class

pinacoidal

Crystal class symbol

-1 (Ci)

Crystal spacegroup

P-1

Refringence type

biaxial

Refringence sign

positive or negative

Birefringence

0.009 - 0.012

Refraction index $n_{\alpha}$

1.528 - 1.576

Refraction index $n_{\beta}$

1.532 - 1.583

Refraction index $n_{\gamma}$

1.538 - 1.588

Dispersion V

weak to relatively weak

Pure color

white

Color

white, gray, bluish white, reddish white, greenish white

Diaphaneity

transparent to subtranslucent

Luster

vitreous

Links

• Wikipedia
• Webmineral database
• Mindat