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
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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