Mineral phase
IMA name
Biotite subgroup
Secondary names
Iron Magnesium Micas
Type
Class
silicate mineral
Compound type
phyllosilicate
Comments
Biotite is now a subgroup including the joins series Annite-Phlogopite, Siderophyllite-Eastonite aand Tetraferriphlogopite-Tetraferriannite
Chemical composition
Formula
$K(Mg,Fe^{2+},Al)_3(Fe^{3+},Al_2,Si)[Si_2O_{10}](OH,F)_2$
Chemical formula
K(Mg,Fe2+,Al)3(Fe3+,Al2,Si)[Si2O10](OH,F)2
Elemental formula
(Mg)0-3 (Fe2+)0-3 (Fe3+)0-1 (Al)0-3 (Si)2-3 O12 (H)0-2 (F)0-2
Isotope mixture type
terrestrial abundance
  Formula Name Family Number min-max Isotope mole fraction State Relevance
$Mg$ Magnesium element 0 - 3 constituent element main
$Fe^{2+}$ Iron(II) cation element 0 - 3 constituent element main
$Fe^{3+}$ Iron(III) cation element 0 - 1 constituent element main
$Al$ Aluminium element 0 - 3 constituent element main
$Si$ Silicon element 2 - 3 constituent element main
$O$ Oxygen element 12 constituent element main
$H$ Hydrogen element 0 - 2 constituent element main
$F$ Fluorine element 0 - 2 constituent element main
Chemical functions
  Function Number
$(OH)^-$ 2
Chemical bonds
  Bond Number
$O^--H$ 1
Oxides comments
variable

Strunz

Class
silicates (09)
Division
phyllosilicate (09.E)
Family
with mica sheets, composed of tetrahedral and octahedral nets (09.EC)
Code
09.EC.20

Dana

Major class
silicates (VIII)
Class
phyllosilicate sheets of six-membered rings (71)
Type
with 2:1 layers (71.02)
Group
Mica group (Biotite subgroup) (71.02.02b)
Phase type
crystalline
Crystal system
monoclinic
Crystal class
prismatic
Crystal class symbol
2/m (C2h)
Crystal spacegroup
C2/m
Polytypes
  • 1M
  • 3T
  • 2M1
Density
2.7 - 3.4 $g/cm^3$
State NTP
solid
Refringence type
biaxial
Refringence sign
negative
Birefringence
0.0258 - 0.066
Refraction index $n_{\alpha}$
1.530 - 1.631
Refraction index $n_{\beta}$
1.557 - 1.69
Refraction index $n_{\gamma}$
1.558 - 1.697
Dispersion V
r > v or r < v
Pure color
various
Color
various
Diaphaneity
transparent to opaque
Luster
various
Links