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Chlorite group

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Chlorite group
Chlorite
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Chlorite

General
Category Mineral group
Formula (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6
Identification
Colour Various shades of green; rarely yellow, red, or white.
Habit Foliated masses, scaley aggregates, disseminated flakes.
System Monoclinic 2/m; with some triclinic polymorphs.
Cleavage Perfect 001
Fracture Lamellar
Hardness 2 - 2.5
Luster Vitreous, pearly, dull
RI 1.57 -1.67
Pleochroism
Streak Pale green to grey
SG 2.6-3.3
Other Folia flexible - not elastic

Chlorite is a group of phyllosilicate minerals often classified as clays. Chlorites can be described by the following four endmembers based on the metals: Mg, Fe, Ni, and Mn.

In addition zinc, lithium and calcium varieties are known. The great range in composition results in considerable variation in physical, optical, and X-ray properties.

Contents

Chlorite structure

The typical general formula is: (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6. This formula emphasises the structure of the group.

Chlorites have a 2:1 sandwich structure (2:1 sandwich layer = tetrahedral-octahedral-tetrahedral = t-o-t...), this is often referred to as a talc layer. Unlike other 2:1 clay minerals, a chlorite's interlayer space (the space between each 2:1 sandwich filled by a cation) is comprised of (Mg2+, Fe3+)(OH)6. This (Mg2+, Fe3+)(OH)6 unit is more commonly referred to as the brucite-like layer, due to its closer resemblance to the mineral brucite (Mg(OH)2). Therefore, chlorite's structure appears as follows:

-t-o-t-brucite-t-o-t-brucite ...
An older classification divided the chlorites into two subgroups: the orthochlorites and leptochlorites. The terms are seldom used and the ortho prefix is somewhat misleading as the chlorite crystal system is monoclinic and not orthorhombic.

Occurrence

Chlorite is commonly found in igneous rocks as an alteration product of mafic minerals such as pyroxenes, amphiboles, and biotite. It is a common alteration mineral associated with hydrothermal ore deposits and commonly occurs with epidote. Chlorite is also a common metamorphic mineral, usually indicative of low-grade metamorphism. It is the diagnostic species of the greenschist facies. It occurs in the quartz, albite, sericite, chlorite, garnet assemblage of pelitic schist.

Members of the Chlorite group:

Clinoclore, pennantite, and chamosite are the most common varieties. Several other sub-varieties have been described.

The name chlorite is from the Greek chloros, meaning "green", in reference to its color.

See also

References

 

From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.

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