Chlorite

Chlorite is a name for a group of closely related minerals that are something between mica and clay. 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.

Fossils
Geologic Features and Processes
Glaciers and Ice
Landforms
Minerals
Rocks
Geology and Society