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Illite

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Formula:
K
 
0.65
Al
 
2.0
[Al
 
0.65
Si
 
3.35
O
 
10
](OH)
 
2
Illite shows a range of compositions (see below). Hydronium may occur in the interlayer region and contribute to charge balance and tetrahedral Al-Si ratios may vary.
System:MonoclinicColour:Gray-white to ...
Hardness:1 - 2
Member of:Mica Group
Name:Named in 1937 by Ralph E. Grim, R. H. Bray, and W. F. Bradley for one of the co-type localities, in Illinois, USA. In the time period the mineral was named, the probable supposition was that the leaching of interlayer K was a consistent feature throughout the mineral's structure and that the alteration was systematic. Grim et al. (1937) noted unusual optical properties of illite as well as a cation exchange capacity of 20-40 meq/100 grams. The literature on "illite" is among the largest in all of mineralogy as it is a widespread component of sediments. Schultz (1978) studied the the development of samples that were 20-60% illite that was randomly interstratified with varying smectite layers (i.e. beidellite or montmorillonite). Stixrude and Peacor (2002) further considered the model of illite relating to its transformation into montmorillonite via two mixed-layer models of crystallization and concluded that charge defect layers influenced the composition of adjacent layers, but which, thermodynamically, should not result in the formation of K-rectorite.
A variety of Muscovite

Mica Group. This clay-like series is essentially a K-deficient muscovite, but frequently contains randomly sequenced montmorillonite/beidellite layers. Illite is dioctahedral, although some references are known which incorrectly refer to "illite" as a similar alteration sequence of trioctahedral micas, although some concomitant Mg substitution in octahedral sites has been suggested. Illite is typically found as extremely fine-grained masses of grayish-white to silvery-gray, sometimes greenish-gray, material. (Paragonite, the Na analog of muscovite, alters in a more or less similar manner as muscovite, and leads to a Na-deficient variety called brammallite.)
Illite is usually an alteration product of muscovite and can be regarded as the mechanism where muscovite may be eventually altered to montmorillonite. During the alteration from muscovite to illite, the structure of illite may become "turbostratified" and the resulting layers of K-filled and K-deficient are randomly mixed, in addition to varying Al-Si substitutions in tetrahedral layers for charge balance.
The Kübler index (KI), earlier called the "illite crystallinity", is a well established method for the characterization of the metamorphic grade of pelites in very low-grade metamorphic environments (Frey, 1987; Guggenheim et al., 2012).
The "Nomenclature of the micas" paper says: "Illite. This name has been used relatively vaguely, and the Subcommittee found it suitable as a series name for a relatively large volume in compositional space, as a counterpart to glauconite."
The formula K0.65Al2.0◻Al0.65Si3.35O10(OH)2 is only a point in this large compositional space, which is regarded "representative" by the Mica Subcommittee of IMA CNMMN. The composition range of illite is described more appropriate by the following formula: K0.6-0.85Al2(Si,Al)4O10(OH)2 or even K0.6-0.85(Al,Mg)2(Si,Al)4O10(OH)2.

Illite is usually an alteration product of muscovite and can be regarded as the mechanism where muscovite may be eventually altered to montmorillonite. During the alteration from muscovite to illite, the structure of illite may become "turbostratified" and the resulting layers of K-filled and K-deficient are randomly mixed, in addition to varying Al-Si substitutions in tetrahedral layers for charge balance.
The Kübler index (KI), earlier called the "illite crystallinity", is a well established method for the characterization of the metamorphic grade of pelites in very low-grade metamorphic environments (Frey, 1987; Guggenheim et al., 2012).

Classification of Illite

Explanation of status:Approved by the IMA Mica Group Subcommittee (Rieder et al.) as a name for a Mica Group series of dioctahedral interlayer-deficient materials, with no end members defined. A compositional range is given, but cannot be reproduced here. The name is for a series in the Mica Group, not an individual species. While it is currently accepted, it is likely that once the species in the series have been better defined it will be replaced by two or more species names.
Dana 7th edition ID:71.2.2.2
Dana 8th edition ID:71.2.2d.2

71 : PHYLLOSILICATES Sheets of Six-Membered Rings
2 : Sheets of 6-membered rings with 2:1 layers
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Type Occurrence of Illite

Type Locality:Maquoketa Shale, Gilead, Calhoun Co., Illinois, USA
Year of Discovery:1937
Geological Setting of type material:The Maquoketa shale, Gilead, Calhoun Co., Illinois, USA; and at Takova, Yugoslavia.
Associated Minerals at type locality:
LimoniteQuartz

Occurrences of Illite

Geological Setting:Found in a wide variety of environments, the materials of this series form by either weathering or hydrothermal alteration of muscovite-phengite; but some is authigenic or could be derived from alteration of K-feldspars or recrystallization of smectites. Common in sediments, clays, marls, shales and some slates. Interstratified illite-smectite converts to illite at depths and mixed-layer ratios have been used to interpret the depth of burial of sediments.

Physical Properties of Illite

Lustre:Waxy, Greasy, Dull, Earthy
Diaphaneity (Transparency):Translucent
Colour:Gray-white to silvery-white, greenish-gray, sometimes stained other hues.
Streak:White
Hardness (Mohs):1 - 2
Tenacity:Elastic
Cleavage:Perfect
Perfect on {001}.
Fracture:Micaceous
Density (measured):2.79 - 2.8 g/cm3
Density (calculated):2.61 g/cm3

Crystallography of Illite

Crystal System:Monoclinic
Class (H-M):2/m - Prismatic
Space Group:B2/m
Cell Parameters:a = 5.19Å, b = 8.95Å, c = 9.95Å
β = 94.87°
Ratio:a:b:c = 0.58 : 1 : 1.112
Unit Cell Volume:V 460.51 ų (Calculated from Unit Cell)
Z:2

Optical Data of Illite

Type:Biaxial (-)
RI values: nα = 1.535 - 1.570 nβ = 1.555 - 1.600 nγ = 1.565 - 1.605
2V:Measured: 5° to 25°, Calculated: 42° to 68°
Maximum Birefringence:δ = 0.030 - 0.035

Chart shows birefringence interference colour range (at 30µm thickness) and does not take into account mineral colouration.
Surface Relief:Low
Dispersion:none

Chemical Properties of Illite

Formula:
K
 
0.65
Al
 
2.0
[Al
 
0.65
Si
 
3.35
O
 
10
](OH)
 
2


Illite shows a range of compositions (see below). Hydronium may occur in the interlayer region and contribute to charge balance and tetrahedral Al-Si ratios may vary.
Simplified for copy/paste:K0.65Al2.0[Al0.65Si3.35O10](OH)2
Essential elements:Al, H, K, O, Si
All elements listed in formula:Al, H, K, O, Si
Analytical Data:Type illite generally had a high phengitic Si:Al ratio (up to 3.51) as well as significant octahedral Mg.

Relationship of Illite to other Species

Member of:Mica Group
Other Members of Group:

- +
Aluminoceladonite
K(Mg,Fe
2+
 
)Al(Si
 
4
O
 
10
)(OH)
 
2
Anandite
(Ba,K)(Fe
2+
 
,Mg)
 
3
((Si,Al,Fe)
 
4
O
 
10
)(S,OH)
 
2
Annite
KFe
2+
3
(AlSi
 
3
O
 
10
)(OH)
 
2
Aspidolite
NaMg
 
3
(AlSi
 
3
O
 
10
)(OH)
 
2
Balestraite
KLi
 
2
V
5+
 
Si
 
4
O
 
12
Biotite
Bityite
LiCaAl
 
2
(AlBeSi
 
2
O
 
10
)(OH)
 
2
Boromuscovite
KAl
 
2
(BSi
 
3
O
 
10
)(OH)
 
2
Brammallite
(Na,H
 
3
O)(Al,Mg,Fe)
 
2
((Si,Al)
 
4
O
 
10
)(OH)
 
2
Celadonite
K(Mg,Fe
2+
 
)Fe
3+
 
(Si
 
4
O
 
10
)(OH)
 
2
Chernykhite
(Ba,Na)(V
3+
 
,Al,Mg)
 
2
((Si,Al)
 
4
O
 
10
)(OH)
 
2
Chromceladonite
K(Mg,Fe
2+
 
)(Cr,Al)(Si
 
4
O
 
10
)(OH)
 
2
Chromphyllite
K(Cr,Al)
 
2
(AlSi
 
3
O
 
10
)(OH,F)
 
2
Clintonite
Ca(Mg,Al)
 
3
(Al
 
3
SiO
 
10
)(OH)
 
2
Eastonite
KMg
 
2
Al(Al
 
2
Si
 
2
O
 
10
)(OH)
 
2
Ephesite
LiNaAl
 
2
(Al
 
2
Si
 
2
O
 
10
)(OH)
 
2
Ferro-aluminoceladonite
K(Fe
2+
 
,Mg)(Al,Fe
3+
 
)(Si
 
4
O
 
10
)(OH)
 
2
Ferroceladonite
K(Fe
2+
 
,Mg)(Fe
3+
 
,Al)(Si
 
4
O
 
10
)(OH)
 
2
Ferrokinoshitalite
(Ba,K)(Fe
2+
 
,Mg)
 
3
(Al
 
2
Si
 
2
O
 
10
)(OH,F)
 
2
Fluorannite
KFe
2+
3
(AlSi
 
3
O
 
10
)(F,OH)
 
2
Fluorophlogopite
KMg
 
3
(AlSi
 
3
O
 
10
)(F,OH)
 
2
Ganterite
(Ba,Na,K)(Al,Mg)
 
2
(AlSi
 
3
O
 
10
)(OH)
 
2
Glauconite
(K,Na)(Fe
3+
 
,Al,Mg)
 
2
((Si,Al)
 
4
O
 
10
)(OH)
 
2
Hendricksite
K(Zn,Mg,Mn
2+
 
)
 
3
(AlSi
 
3
O
 
10
)(OH)
 
2
Kinoshitalite
(Ba,K)(Mg,Mn
2+
 
,Al)
 
3
(Al
 
2
Si
 
2
O
 
10
)(OH)
 
2
Lepidolite
Luanshiweiite
KLiAl
 
1.5
□0
 
.5
(Si
 
3.5
Al
 
0.5
)O
 
10
(OH,F)
 
2
Margarite
CaAl
 
2
(Al
 
2
Si
 
2
O
 
10
)(OH)
 
2
Masutomilite
(K,Rb)(Li,Mn
3+
 
,Al)
 
3
(AlSi
 
3
O
 
10
)(F,OH)
 
2
Montdorite
(K,Na)
 
2
(Fe
2+
 
,Mn
2+
 
,Mg)
 
5
(Si
 
4
O
 
10
)
 
2
(OH,F)
 
4
Muscovite
KAl
 
2
(AlSi
 
3
O
 
10
)(OH)
 
2
Nanpingite
CsAl
 
2
(AlSi
 
3
O
 
10
)(OH,F)
 
2
Natro-glauconite
(Na,K)(Fe
3+
 
,Al,Mg)
 
2
((Si,Al)
 
4
O
 
10
)(OH)
 
2
Norrishite
KLiMn
3+
2
(Si
 
4
O
 
10
)O
 
2
Oxykinoshitalite
(Ba,K)(Mg,Ti,Fe
3+
 
,Fe
2+
 
)
 
3
((Si,Al)
 
4
O
 
10
)(O,OH,F)
 
2
Oxyphlogopite
K(Mg,Ti,Fe)
 
3
[(Si,Al)
 
4
O
 
10
](O,F)
 
2
Paragonite
NaAl
 
2
(AlSi
 
3
O
 
10
)(OH)
 
2
Phengite
Phlogopite
KMg
 
3
(AlSi
 
3
O
 
10
)(OH,F)
 
2
Polylithionite
KLi
 
2
Al(Si
 
4
O
 
10
)(F,OH)
 
2
Preiswerkite
NaMg
 
2
Al(Al
 
2
Si
 
2
O
 
10
)(OH)
 
2
Roscoelite
K(V
3+
 
,Al)
 
2
(AlSi
 
3
O
 
10
)(OH)
 
2
Shirokshinite
KNaMg
 
2
(Si
 
4
O
 
10
)F
 
2
Shirozulite
K(Mn
2+
 
,Mg)
 
3
(AlSi
 
3
O
 
10
)(OH)
 
2
Siderophyllite
KFe
2+
2
Al(Al
 
2
Si
 
2
O
 
10
)(OH)
 
2
Sokolovaite
CsLi
 
2
Al(Si
 
4
O
 
10
)F
 
2
Suhailite
(NH
 
4
)Fe
2+
3
(AlSi
 
3
O
 
10
)(OH)
 
2
Tainiolite
KLiMg
 
2
(Si
 
4
O
 
10
)F
 
2
Tetraferriannite
KFe
2+
3
((Fe
3+
 
,Al)Si
 
3
O
 
10
)(OH)
 
2
Tetraferriphlogopite
KMg
 
3
(Fe
3+
 
Si
 
3
O
 
10
)(OH,F)
 
2
Tobelite
(NH
 
4
,K)Al
 
2
(AlSi
 
3
O
 
10
)(OH)
 
2
Trilithionite
K(Li,Al)
 
3
(AlSi
 
3
O
 
10
)(F,OH)
 
2
Voloshinite
Rb(LiAl
 
1.5
 
1.5
)(Al
 
0.5
Si
 
3.5
)O
 
10
F
 
2
Wonesite
(Na,K)(Mg,Fe,Al)
 
6
((Al,Si)
 
4
O
 
10
)
 
2
(OH,F)
 
4
Yangzhumingite
KMg
 
2.5
(Si
 
4
O
 
10
)F
 
2
Zinnwaldite
KLiFe
2+
 
Al(AlSi
 
3
O
 
10
)(F,OH)
 
2
Related Minerals - Dana Grouping):

- +
71.2.2.1Muscovite
KAl
 
2
(AlSi
 
3
O
 
10
)(OH)
 
2
71.2.2.5Glauconite
(K,Na)(Fe
3+
 
,Al,Mg)
 
2
((Si,Al)
 
4
O
 
10
)(OH)
 
2
71.2.2.13Aspidolite
NaMg
 
3
(AlSi
 
3
O
 
10
)(OH)
 
2

Other Names for Illite

Synonyms:
GlimmertonKillianite
Other Languages:
Dutch:Illiet
Esperanto:Ilito
Estonian:Illiit
French:Illite
German:Illit
Hebrew:איליט
Italian:Illite
Japanese:イライト
Polish:Illit
Russian:Иллит
Simplified Chinese:伊利石
Spanish:Illita
Ukrainian:Іліт
Varieties:
Al-illite-hydromicaAmmersooiteAvaliteIllite JadeLeverrierite

Polytypes for Illite

Polytypes:
Illite-1MIllite-1MdIllite-2M

Other Information

Fluorescence in UV light:Not fluorescent in UV
Health Warning:No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.

References for Illite

Reference List:

- +
Grim, Ralph E., Bray, R. H., and Bradley, W. F. (1937) The Mica in Argillaceous Sediments, American Mineralogist, v. 22, p. 813-829.
Gaudette, H. E. (196x) The Nature of Illite, Thirteenth Conference on Clay and Clay Minerals, p. 1-35.
Eslinger, E., P. Highsmith, D. Albers, and B. deMayo (1979), Role of iron reduction in the conversion of smectite to illite in bentonites in the disturbed belt, Montana. Clays and Clay Minerals, 27, 327-338.
Nadeau, P. & D. Bain (1986): Composition of some smectites and diagenetic illitic clays and implications for their origin. Clays and Clay Minerals, 34, 455-464.

Clays and Clay Minerals
(1993): 41, 45, 389.

Rieder, et.al.
(1998) (IMA Mica Group Subcommittee Report.) Canadian Mineralogist, 36: 905-912.
Drits, V.A., L.G. Dainyak, F. Muller, G. Besson, and A. Manceau (1997a), Isomorphous cation distribution in celadonites, glauconites and Fe-illites determined by infrared, Mössbauer and EXAFS spectroscopy: Clay Minerals: 32: 153-180.
Gaines, Richard V., H. Catherine, W. Skinner, Eugene E. Foord, Brian Mason, Abraham Rosenzweig, Vandall T. King (1997), Dana's New Mineralogy : The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana: 1472.
Cuadros, J., and S.P. Altaner (1998), Compositional and structural features of the octahedral sheet in mixed layer illite-smectite from bentonites: European Journal of Mineralogy: 10: 111-124.
Stixrude, Lars and Peacor, Donald Ralph (2002) First Principles Study of Illite-Smectite and Implications for Clay Minerals Systems, Nature, v. 420 (November 4): 165-168.
Guggenheim, S. et al. (2002): Clays Clay Minerals 50, 406-409.
Pironon, J., Pelletier, M., De Donato, P. and Mosser-Ruck, R. (2003) Characterization of smectite and illite by FTIR spectroscopy of interlayer NH4 + cations. Clay Minerals: 38: 201-211.
Sainz-Diaz, C.I., E.J. Palin, M.T. Dove, and A. Hernandez-Laguna (2003a), Monte Carlo simulations of ordering of Al, Fe and Mg cations in the noctahedral sheet of smectites and illites: American Mineralogist: 88: 1033-1045.
Sainz-Diaz, C.I., E.J. Palin, A. Hernandez-Laguna, and M.T. Dove (2003b) Octahedral cation ordering of illite and smectite. Theoretical exchange potential determination and Monte Carlo simulation: Physics and Chemistry of Minerals: 30: 382-392.
Dainyak, Lidia G., Victor A. Drits, Bella B. Zviagina, and Holger Lindgreen (2006), Cation redistribution in the octahedral sheet during diagenesis of illite-smectites from Jurassic and Cambrian oil source rock shales: American Mineralogist: 91: 589-603.

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  • Localities for Illite

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