Chlorite - (Mg,Fe²⁺,Fe³⁺,Mn,Al)₁₂[(Si,Al)₈O₂₀](OH)₁₆

Chlorite is the group name for about 10 related minerals. However, the term Chlorite can be used both to describe the group in general, or as a specific term to describe any green member of the Chlorite group whose exact identity is not practical to be determined.

The Chlorite group minerals are mostly monoclinic (also triclinic or orthorhombic) micaceous phyllosilicate minerals with a structure consisting of T-O-T layers with two layers having their silicate tetrahedral apices pointing towards each other, separated by an interlayer that may be simple octahedrally coordinated cations or which may be a brucite-like layer of two sheets of closely packed OH groups with the interstices between sheets providing the octahedral cordination site.

Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice:

♦ 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 species are known. The great range in composition results in considerable variation in physical, optical, and X-ray properties. Similarly, the range of chemical composition allows chlorite group minerals to exist over a wide range of temperature and pressure conditions. For this reason chlorite minerals are ubiquitous minerals within low and medium temperature metamorphic rocks, some igneous rocks, hydrothermal rocks and deeply buried sediments.

Chlorite is an important constituent of many contact and regional metamorphic rocks of low to medium grade, usually with temperatures of to 400°C and pressures up to a few Kb. It also found in amygdules, fractures in altered volcanic rocks, hydrothermal vein deposits and soils.
Chlorite is often found with biotite, garnet, staurolite, andalusite, muscovite, chloritoid, and cordierite in pelitic rocks. In mafic rocks, it occurs with talc, serpentine, actinolite, hornblende, epidote, and garnet. It can also be found with feldspars, quartz, calcite, dolomite, olivine, plagioclase, rutile, ilmenite, titanite, magnetite, chromite, sulfides, zircon, and zeolites.

Chlorite forms by the alteration of mafic minerals such as pyroxenes, amphiboles, biotite, staurolite, cordierite, garnet, and chloritoid. Chlorite can also occur as a result of hydrothermal alteration of any rock type, where recrystallization of clay minerals or alteration of mafic minerals produce chlorite.

Optical properties:
• Color: colourless ore green, pale green
• Habit: tabular crystals with pseudo-hexagonal shape
• Cleavage: (001) perfect
Interference colors: very weak (brown, deep green, gray), usually with anomalous deep berlin blue
• Relief: low to moderate

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Biotite partially alterd by chlorite with berlin blue interference colors in a dacite (Greece). XPL image , 10x (Field of view = 2mm)	Biotite partially alterd by chlorite with berlin blue interference colors in a dacite (Greece). XPL image , 10x (Field of view = 2mm)	Biotite partially alterd by chlorite with berlin blue interference colors in a dacite (Greece). XPL image , 10x (Field of view = 2mm)
Biotite partially alterd by chlorite with berlin blue interference colors in a Dacite (Greece). XPL image , 10x (Field of view = 2mm)	Biotite partially alterd by chlorite with berlin blue interference colors in a dacite (Greece). XPL image , 10x (Field of view = 2mm)	Biotite partially alterd by chlorite with berlin blue interference colors in a dacite (Greece). XPL image , 10x (Field of view = 2mm)
Biotite partially alterd by chlorite with berlin blue interference colors in a dacite (Greece). XPL image , 10x (Field of view = 2mm)