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(also pennine; named after the Pennine Alps, where the mineral was first discovered), a mineral of the chlorite group, widely distributed in metamorphic rocks, with the approximate chemical formula (Mg, Fe)5•Al[AlSi3O10](OH)8. Penninite crystallizes in the monoclinic system to form minute, flat, micaceous crystals with perfect cleavage. It generally occurs as solid, scaly masses or as soft, flexible individual scales. Its color varies from green to blackish green, except for the chromium-rich penninites, such as rhodochrome and kammererite, which are pink, violet, or carmine. Penninite has a hardness of 2.0–2.5 on Mohs’ scale and a density of 2,600–2,850 kg/m3. It is formed as a secondary mineral during the metamorphism of ferromagnesian silicates, such as olivine, enstatite, hornblende, and biotite.