Mineral phase
IMA name
Albite-Anorthite series
Secondary names
Plagioclase feldspar
Type
Class
silicate mineral
Compound type
tektosilicate
Comments
Plagioclase is a wider series $(Na,Ca)(Si,Al)_4O_8$ of the feldspar group
Compositional phase diagram of the different minerals of the feldspar solid solution (Wikipedia - By Muskid - Own work, CC BY-SA 4.0)
Chemical composition
Formula
$(Na,Ca)(Al,Si)AlSi_2O_8$
Chemical formula
((Na)x(Ca)1-x)((Al)1-x(Si)x)AlSi2O8
Elemental formula
(Na)0-1 (Ca)0-1 (Al)1-2 (Si)2-3 O8
Isotope mixture type
terrestrial abundance
  Formula Name Family Number min-max Isotope mole fraction State Relevance
$Na$ Sodium element 0 - 1 constituent element main
$Ca$ Calcium element 0 - 1 constituent element main
$Al$ Aluminium element 1 - 2 constituent element main
$Si$ Silicon element 2 - 3 constituent element main
$O$ Oxygen element 8 constituent element main
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
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
Molar mass
263.02 - 277.41 $g/mol$
Density
2.61 - 2.76 $g/cm^3$
State NTP
solid
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