HUMITE

This article appears in Volume V13, Page 885 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: HOR-I25

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

VESUVIUS (also Vesevus in ancient poets)

; 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

JERSEY
JERSEY, EARLS OF

. 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

SERIES (a Latin word from serere, to join)

, 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. S.)

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