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 51:9, 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 6-6zi 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 x6pbpos, " 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 Orebro, Sweden, similar crystals (of chondrodite) occur em-bedded 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 irpoXEyecv, " to foretell ") . (L . J .