HUMITE, a group of three related minerals consisting of basic magnesium fluo-silicates, with the formulae : chondrodite, humite, clinchumite, Mg, [ Mg (F,OH) ] 2 [ Humite crystallizes in the orthorhombic and the two others in the monoclinic system, but be tween 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. Such a relation between the crystallographic constants and the chemical composition is commonly known as a morpho tropic relation.
The three minerals are strikingly similar in appearance, and can only be distinguished by the goniometric measurement of the com plex crystals. They are honey-yellow to brown or red in colour, and have a vitreous to resinous lustre; hardness 6-6.5, specific gravity 3.1-3.2. The name humite, after Sir Abraham Hume was given 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 xovBpos, "a grain") was a name early (1817) in use for granular forms of these minerals found embedded in crystalline limestones in Swe den, Finland and at several places 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 embedded in galena and chalcopyrite.
