Chlorite group

Lore & Background

Chlorite minerals form blue-green crystals resembling mica, but their plates are flexible rather than elastic and are less easily pulled apart. The structure is described as TOT-O, consisting of alternating talc-like TOT layers and positively charged brucite-like O layers. Unlike talc, the TOT layers in chlorite carry a negative charge due to aluminium substitution, and they are bound by the O layers rather than by individual cations as in mica. Chlorite is considered a nonswelling clay mineral with low cation exchange capacity.

Reader's Guide

Chlorite is a common mineral found in metamorphic, igneous, and sedimentary rocks. It is an important rock-forming mineral in low- to medium-grade metamorphic rocks formed from mafic or pelitic protoliths. In igneous rocks, it typically occurs as a secondary mineral from alteration of mafic minerals such as biotite, hornblende, pyroxene, or garnet. The green color of many igneous rocks, slates, and schists is due to disseminated chlorite. G.M. At higher temperatures, chlorite is destroyed by reactions producing biotite, muscovite, and quartz. Chlorite also forms by hydrothermal propylitic alteration and may be stable in mantle peridotite above subducting ocean lithosphere. It has no major industrial uses but appears in decorative stone and roofing granules.

Did You Know?

• Chlorite minerals do not contain the element chlorine; the name comes from the Greek 'chloros' meaning 'green'.
• A complete solid solution series exists between magnesium-rich clinochlore and iron-rich chamosite.
• Chlorite is a nonswelling clay mineral with relatively low cation exchange capacity.
• The glassy rims of pillow basalt on the ocean floor are often altered to pure chlorite by exchange with seawater.