Moore, Paul B. Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois.
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A homologous series of magnesium nesosilicate minerals having the general composition Mg2n + 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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