Lepidomelane

lepidomelane [‚lep·ə·dō′me‚lān]

(mineralogy)

A black variety of biotite that is characterized by the presence of large amounts of ferric iron. Also known as iron mica.

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Lepidomelane 

a mineral of the trioctahedral mica group; an iron-rich variety of biotite. The approximate chemical composition of lepidomelane is K(Fe, Mg)₃[AlSi₃O₁₀] • (OH)₂. It is possible that part of the Al in the lamellae, as well as the Fe²⁺, is replaced by Fe³⁺ (up to 20 percent Fe₂O₃). Lepidomelane crystallizes in the monoclinic system. Its physical properties resemble those of biotite, and its color is black. Lepidomelane has a hardness of ~ 3 on Mohs’ scale and a density of 3,000–3,200 kg/m³. It occurs primarily in igneous rocks rich in iron and deficient in magnesium; sometimes it is found in alkali granites and nepheline syenites in association with aegirite and hornblende. Large crystals of lepidomelane are found in Hastings (Ontario, Canada), on Mount Brooks (Seward Peninsula, Alaska), in the alkali rocks on the Kola Peninsula, and in the southern Urals.