Part III. Ilvaite from El Guaricho, Michoacán
Keywords:
crystals, metasomatism, gravity, oxidation, magmatic fluidsAbstract
Ilvaita, from the El Guaricho area in Michoacán, was studied mineralogically. The mineral appears as prismatic, jet-black, magnetic crystals, orthorhombic, with a space group of Pbmn, a = 8.850Å, b = 12.990, c = 5.905, Z = 3, with a specific gravity of 3.99, and the formula Ca1.0234Fe²⁺1.5421Fe³⁺1.0960Mg0.0994Mn0.1911(SiO₄)₂(OH). Oxidation of Fe²⁺ begins at 590°C, followed by a rapid exothermic destruction of the lattice into magnetite, hematite, cristobalite, and rankinite at 770°C. Ilvaita is found in "boleos" near the contact between sedimentary limestone and intrusive diabase, and it is estimated that it may have crystallized from residual magmatic liquids rich in Fe, Ca, Mn, Cu, etc., either contemporaneously with or slightly after the augite in the diabase. Its formation may also have been possible through contact metasomatism.
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