Chlorite group: Encyclopedia - Chlorite group

Chlorite group

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.

• Clinochlore: (Mg₅Al)(AlSi₃)O₁₀(OH)₈
• Chamosite: (Fe₅Al)(AlSi₃)O₁₀(OH)₈
• Nimite: (Ni₅Al)(AlSi₃)O₁₀(OH)₈
• Pennantite: (Mn,Al)₆(Si,Al)₄O₁₀(OH)₈

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.

Chlorite group - 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:

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.

Chlorite group - 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.

Chlorite group - Members of the Chlorite group:

• Baileychlore (Zn,Fe⁺²,Al,Mg)₆(Al,Si)₄O₁₀(O,OH)₈
• Chamosite (Fe,Mg)₅Al(Si₃Al)O₁₀(OH)₈
• Clinochlore (Mg,Fe²⁺)₅Al(Si₃Al)O₁₀(OH)₈
• Cookeite LiAl₄(Si₃Al)O₁₀(OH)₈
• Donbassite Al₂[Al_2.33][Si₃AlO₁₀](OH)₈
• Gonyerite (Mn,Mg)₅(Fe⁺³₎₂Si₃O₁₀(OH)₈
• Nimite (Ni,Mg,Al)₆(Si,Al)₄O₁₀(OH)₈
• Odinite (Fe,Mg,Al,Fe,Ti,Mn)_2.4(Al,Si)₂O₅OH₄
• Orthochamosite (Fe⁺²,Mg,Fe⁺³)₅Al(Si₃Al)O₁₀(O,OH)₈
• Pennantite (Mn₅Al)(Si₃Al)O₁₀(OH)₈
• Ripidolite (Mg,Fe,Al)₆(Al,Si)₄O₁₀(OH)₈
• Sudoite Mg₂(Al,Fe)₃Si₃AlO₁₀(OH)₈

Clinoclore, penninite, 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

• List of minerals

Adapted from the Wikipedia article "Chlorite group", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki