Humite

Humite, a group of minerals consisting of basic magnesium fluo-silicates, with the following formulae:—Chondrodite, Mg3[Mg(F, OH)]2[SiO4]2; Humite, Mg5[Mg(F, OH)]2[SiO4]3; Clinohumite, Mg7[Mg(F, OH)]2[SiO4]4. Humite crystallizes in the orthorhombic and the two others in the monoclinic system, but between them there is a close crystallographic relation: the lengths of the vertical axes are in the ratio 5:7:9, and this is also the ratio of the number of magnesium atoms present in each of the three minerals. These minerals are strikingly similar in appearance, and can only be distinguished by the goniometric measurement of the complex crystals. They are honey-yellow to brown or red in colour, and have a vitreous to resinous lustre; the hardness is 6-6½, and the specific gravity 3.1-3.2. Further, they often occur associated together, and it is only comparatively recently that the three species have been properly discriminated. The name humite, after Sir Abraham Hume, Bart. (1749-1839), whose collection of diamond crystals is preserved at Cambridge in the University museum, was given by the comte de Bournon in 1813 to the small and brilliant honey-yellow crystals found in the blocks of crystalline limestone ejected from Monte Somma, Vesuvius; all three species have since been recognized at this locality. Chondrodite (from χόνδρος, “a grain”) was a name early (1817) in use for granular forms of these minerals found embedded in crystalline limestones in Sweden, Finland and at several place in New York and New Jersey. Large hyacinth-red crystals of all three species are associated with magnetite in the Tilly Foster iron-mine at Brewster, New York; and at Kafveltorp in Örebro, Sweden, similar crystals (of chondrodite) occur embedded in galena and chalcopyrite.

The relation mentioned above between the crystallographic constants and the chemical composition is unique amongst minerals, and is known as a morphotropic relation. S. L. Penfield and W. T. H. Howe, who in 1894 noticed this relation, predicted the existence of another member of the series, the crystals of which would have a still shorter vertical axis and contain less magnesium, the formula being Mg[Mg(F, OH)]2SiO4; this has since been discovered and named prolectite (from προλέγειν, “to foretell”).

(L. J. S.)

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