Mineral phase
IMA name
Illite
Type
Class
silicate mineral
Compound type
phyllosilicate
Chemical composition
Formula
$(K_{0.6-0.85})(Al,Mg)_2(Si,Al)_4O_{10}(OH)_2$
Chemical formula
K0.6-0.85(Al,Mg)2(Si,Al)4O10(OH)2
Elemental formula
(Al)1-3 (K)0.6-0.85 (Mg)0-1 (Si)3-4 O12 H2
Isotope mixture type
terrestrial abundance
  Formula Name Family Number min-max Isotope mole fraction State Relevance
$Al$ Aluminium element 1 - 3 constituent element main
$K$ Potassium element 0.6 - 0.85 constituent element main
$Mg$ Magnesium element 0 - 1 constituent element main
$Si$ Silicon element 3 - 4 constituent element main
$O$ Oxygen element 12 constituent element main
$H$ Hydrogen element 2 constituent element main
Composition comments
Generally considered as K-deficient muscovite with more Si. Si and Al proportions are very variables and Mg is generally minor. Common impurities:Fe
Chemical functions
  Function Number
$(OH)^-$ 2
Chemical bonds
  Bond Number
$O^--H$ 1
Oxides
Formula $Al_{2}O_{3}$ $K_{2}O$ $MgO$ $SiO_{2}$ $H_{2}O$
Mass fraction (%)

Strunz

Class
silicates (09)
Division
phyllosilicate (09.E)
Family
Phyllosilicates with mica sheets, composed of tetrahedral and octahedral nets (09.EC)
Code
09.EC.60

Dana

Major class
silicates (VIII)
Class
phyllosilicate sheets of six-membered rings (71)
Type
with 2:1 layers (71.02)
Group
Mica group (Hydromica subgroup) (71.02.02d)
Code
71.02.02d.02
Phase type
crystalline
Crystal system
monoclinic
Crystal class
prismatic
Crystal class symbol
2/m (C2h)
Crystal spacegroup
C2/m
Comments
Illite has different polytypes: Illite-1M, Illite-1Md, Illite-2M1, Illite-2M2, Illite-3T
Molar mass
389.34 $g/mol$
Density
2.6-2.9 $g/cm^3$
State NTP
solid
Refringence type
biaxial
Refringence sign
negative
Birefringence
0.0300-0.0350
Refraction index $n_{\alpha}$
1.535-1.57
Refraction index $n_{\beta}$
1.555-1.6
Refraction index $n_{\gamma}$
1.565-1.605
Dispersion V
none
Pure color
white
Color
White,Gray-white, Silvery-white, Greenish-gray
Diaphaneity
translucent
Luster
earthy
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