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

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

Links

• Wikipedia
• Webmineral database
• Mindat