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Chlorite

The Silicate Minerals

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Chlorite is a soft, flaky mineral that is something between mica and clay. It often accounts for the green color of metamorphic rocks. (more below)
Hydrous metal silicate
Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)
Chlorite is usually green, soft (Mohs hardness 2 to 2.5), with a pearly to glassy luster and micaceous or massive habit.

Chlorite is very common in low-grade metamorphic rocks like slate, phyllite and greenschist. However, chlorite can appear in higher-grade rocks as well. You'll also find chlorite in igneous rocks as an alteration product, where it sometimes occurs in the shape of the crystals it replaces (pseudomorphs). Chlorite looks like a mica, but when you split off its thin sheets, they are flexible but not elastic—that is, they bend but do not spring back—whereas mica is always elastic.

Chlorite's molecular structure is a stack of sandwiches consisting of a silica layer between two metal oxide (brucite) layers, with an extra brucite layer laced with hydroxyl between the sandwiches. The general chemical formula reflects the wide range of compositions in the chlorite group: (R2+,R3+)4–6(Si,Al)4O10(OH,O)8 where R2+ can be Al, Fe, Li, Mg, Mn, Ni or Zn (usually Fe or Mg) and R3+ is usually Al or Si.

Other Metamorphic Minerals

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