Chlorite group of minerals are phyllosilicates (sheet silicates) that commonly form during low-grade metamorphism and the alteration of igneous rocks. They are characteristic of rocks such as greenschist, which develops when basalt or other volcanic rocks undergo metamorphism.
Chlorite minerals have variable compositions, with magnesium (Mg), iron (Fe), aluminum (Al), and silicon (Si) often substituting for one another in the crystal structure. A solid-solution series exists between the two main end members: clinochlore (magnesium-rich) and chamosite (iron-rich). Other less common types may contain manganese, zinc, lithium, or calcium.