Corundum Enlarge
June 1 1812 published by Jas Sowerby London.
Exotic Mineralogy
XLIV
Argilla durissima

Corundum

  • Syn.
    • Imperfect Corundum, Bournon.
    • Adamantine Spar, Kirw. 1. 335.
    • Demant Spath, Emmerl. 1. 9.

This plate exhibits several of those varieties of Corundum to which that appellation most correctly belongs, and which have been proved by analysis, a.s well as by their physical characters, to be the same as the Ruby, Sapphire, &c. only less perfectly crystallized. The external form of the crystals of this variety is generally very imperfect, the surface rugged and dull, but in some specimens the form is better defined, and the surface somewhat smoother. Such specimens approach towards the more opaque and rugged Rubies. The fracture of this variety is always very distinctly foliated, the laminas parallel to an obtuse rhomb of 94° and 86°. It frequently also breaks in the direction of the terminal plane of the prism, as in fig. 2 and the right hand figure at the bottom of tab. 45; both these fractures may sometimes be observed in the Ruby, although the more perfect cohesion of the particles in that commonly prevents their being obtained. The hardness is much tlie same in all the varieties. The mean Specific Gravity of the present variety taken from 33 specimens by the Count de Bournon was 3.931, the lightest was3.875, and the heaviest 3.981. The colour is very variable, as may be seen by the figures.

There are two kinds of stone in which the common Corundum is found; one of them consists of a peculiar kind of Feldspar, which contains an extraordinary quantity of Lime and is accompanied by Hornblende, Epidote, Quartz, Garnets, Fibrolite, Mica, Talc, and Suboxide of Iron. This matrix is sometimes found in a state of decomposition, when the lime, having been previously separated, perhaps by water, forms a kind of cement which surrounds broken pieces of Comndum. The other kind of matrix is shown in the next plate.

No. 1 is the primitive crystal; it is imbedded in Feldspar of a white colour mixed with Lime. No. 2 a fragment showing the fracture upon the solid angle, and corresponding with the modification, No. 1, upon the last plate. No. 3 a remarkably well defined crystal, nearly similar in form to No. 3 of the last plate, and of a colour approaching to that of the Oriental Topaz, a brownish yellow, the terminal plane is deeply striated, as in No. 10 of the last plate. Nos. 4, 5, and 6, arc varieties of the hexaëdral prism; and Nos. 7 and 8 show the hexaëdral pyramid. No. 9 is modified in a manner similar to No.18 of the last plate, the planes corresponding to the obtuse rhomb being striated in a like manner.

Of those Nos. 1, 2, 4, 6, and 9, are from the Carnatic, and Nos. 3, 5, 7, and 10, from Ava. They are all, excepting No 2 and No. 8, in the select cabinet of the Count de Bournon; No. 8, which is from Malabar, is in the British Museum.

Analysis of Common Corundum, from the Carnatic.
Alumina 91.0
Silex 5.0
Iron 1.5
Loss 2.5
100.0
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