Illite

Poços de Caldas, Minas Gerais, Southeast Region, Brazil
© Martins da Pedra

Show Illite Photos (6)

Formula:

0.65

2.0

[][Al

0.65

Si,

3.35

](OH)

System:

Monoclinic

Colour:

Grey-white to ...

Hardness:

1 - 2

Name:

Named in 1927 for one of the co-type localities, in Illinois, USA.

Mica Group. This clay-like series is typically found as extremely fine-grained masses of greyish-white to silvery-grey, sometimes greenish-grey, material.

Classification of Illite

IMA status:

Approved 1998

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.

Strunz 8th edition ID:

8/.-

Nickel-Strunz 10th (pending) edition ID:

9..

9 : SILICATES (Germanates)
:
:

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.

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.

Physical Properties of Illite

Lustre:

Pearly

Diaphaneity (Transparency):

Translucent

Colour:

Grey-white to silvery-white, greenish-gray, sometimes stained other hues.

Streak:

White

Hardness (Mohs):

1 - 2

Tenacity:

Elastic

Cleavage:

Perfect
Perfect on {001}.

Fracture:

Micaeous

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 (B1 1 2/m) [C2/m] {C1 2/m 1}

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 Å³ (Calculated from Unit Cell)

Optical Data of Illite

Type:

Biaxial (-)

RI values:

n_α = 1.535 - 1.570 n_β = 1.555 - 1.600 n_γ = 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:

0.65

2.0

[][Al

0.65

Si,

3.35

](OH)

Essential elements:

Al, H, K, O, Si

All elements listed in formula:

Relationship of Illite to other Species

9..

Lepidolite

9..

Tetranatrolite

Related Minerals - Dana Grouping):

71.2.2.1

Muscovite

KAl

[(OH)

|AlSi

]

71.2.2.5

Glauconite

0.08

1.33

0.67

[][Al

0.13

3.87

](OH)

71.2.2.13

Aspidolite

NaMg

[(OH)

|AlSi

]

Other Names for Illite

Synonyms:

Esperanto names:

German names:

Japanese names:

Russian names:

Иллит

Spanish names:

Varieties:

Polytypes for Illite

Polytypes:

Illite-1M

Illite-1Md

Illite-2M

Other Information

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:

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., Canadian Mineralogist (1998), (IMA Mica Group Subcommittee Report.): 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 (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.

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.