Humite

Article By:

Moore, Paul B. Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois.

Last reviewed:November 2019

DOI:https://doi.org/10.1036/1097-8542.325400

A homologous series of magnesium nesosilicate minerals having the general composition Mg_{2n + 1}(SiO₄)_n(F,OH)₂. The known species include norbergite (n = 1), chondrodite (n = 2), humite (n = 3), and clinohumite (n = 4). They are structurally related to forsterite olivine, Mg₂SiO₄, and brucite, Mg(OH)₂. All are based on hexagonal close-packed oxygen and fluorine atoms, the Mg atoms occupying octahedral interstices and the Si atoms occupying tetrahedral interstices. The hexagonal close-packed repeat distance is approximately 0.47 nm in these minerals. Forsterite, norbergite, and humite are orthorhombic; chondrodite and clinohumite are monoclinic; brucite is trigonal. Manganese analogs of these minerals occur as pink grains in metamorphosed manganese ores derived from preexisting siliceous carbonates and sedimentary manganese oxides. Other cations which can occur as substituents are Fe²⁺, Ca²⁺, Al³⁺, and Ti⁴⁺. Titanoclinohumite, for example, a high-pressure phase, has been found in deep-seated rocks. It may be one of the storage minerals for water at depth.

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