Scotch Corundum Enlarge
April 1. 1804. Publiſhed by Ja.s Sowerby. London.
British Mineralogy
LXIX
Argilla durissima

Scotch Corundum

  • Class 2. Earths.
  • Order 1. Homogeneous.
  • Gen. 2. Argil.
  • Spec. 8. Corundum.
  • Gen. Char. Unctuous to the touch. Easily diffusible in water. Adheres to the tongue. Spec. Grav. 2. Kirw. Combines difficulty with acids, forming with most of them deliquescent salts, soluble in borax. Bab.
  • Spec. Char. Nearly pure argil, hardest of all minerals except the diamond. Divisible parallel to a rhomb, the angles of which are 86° 26′. 93° 34′.

This curious substance was sent me among other things from a dealer at Aberdeen, under the name of Red Schorle from Achen-door. I figure it here, because it is a substance which appears to be new to British writers. Upon inquiry I found it was very little known, nor was it to be found in any mineralogical collection in London, nor scarcely in Scotland. I therefore was glad to present a few of my friends with it. Even Mr. Jameson had not previously obtained it. From him I hope for a good account of it. It occurs in long columns or bars from an eighth to three quarters of an inch thick, mostly confused, often diverging and with transverse flaws, having the matrix intervening abruptly. Its fractures are longitudinal and splintery. The columns are four-sided, with faces replacing the edges in the centre of the angles: on one, two, or more sides, the ends approach a pyramid (in such as I have seen) with four rhomboidal faces. Among a tolerable quantity, I found very few with crystallized terminations, as figured: the faces however are very distinct.

We find this fossil has been taken for a rubellite, and Kirwan’s description in a great measure accords with that idea. See Kirw. v. 1. 288. but in many respects it has been confounded with the titanite of Kirwan. See his description. May the radiating variety be the substance of which Macquart says the garnets are formed? He describes it as consisting of straight fibres diverging from a common centre. See Kirw. v. 1. 261. Its common appearance resembles garnet much, but it is not fusible by the blowpipe, whereas garnet is fusible into a black enamel.

Kirwan mentions red schorl, p. 271, and says rubellites are also called. Another substance resembling this, according to the short description of Mr. Kirwan, was found by Morveau in Poitou, v. 1. 336, which he preseumed to be adamantine spar. Again, as Haüy observes, another mentioned by M. Morveau, found in Le Forez, resembles it greatly, and which is of great harness. See Kirw. 337.

Hardness of ours is nearly the same as that of spinelle. We found that the harder spinelles would scratch it; but the softer ones are scratched by it. This seems undoubtedly the “Spath adamantin d’un rouge violet” of Bournon, which he described in the year 1789 from specimens found in Le Forez, (Journal de Physique 453.) and now considers as a variety of corundum. Other authors have had similar idea. We here subjoin a part of this description: see Phil. Trans. for 1802, 323. where quoting Haüy, v. 4. 562. who observes “that it scratches quartz; that its specific gravity is 3.165, and that it is infusible by means of the blowpipe;” Bournon observes, “that it is red with a purplish tinge*; that the appearance of the substance was entirely different from that of felspar; and that where it came in contact with the felspar it seemed to mix itself with it in such an insensible manner, that after having sawed and polished a piece composed partly of felspar and partly of the substance here spoken of, it was impossible by the eye to distinguish exactly where the felspar began, or where the other substance terminated.” Ours is readily distinguished from felspar, which it invests occasionally so that it is formed round it like a tube, see the middle figure at the bottom: it is also often running among it in the directions of the fragments, often passing abruptly across it. The nearest approach to mixing insensibly is by the fibres, which in ours are however sufficiently distinct. The Count continues to observe, “that the pieces he had collected varied considerably in their degree of hardness, although all of them were harder than the felspar usually is, for many of these pieces would scarcely scratch felspar; whereas others could scarcely be scratched by the greatest number of gems, or precious stones. The characters of the last-mentioned or hardest pieces appeared to be very similar to those of the imperfect corundum from China, a crystal of which Romé de Lisle had sent him a short time before. The above observations, joined to the remarkable manner in which this substance was mixed with the felspar, made him adopt the erroneous opinion mentioned by the abbé Haüy in his observations upon corundum; namely that this substance might be nothing more than a dense variety of felspar. He soon however entirely gave up this idea, after he had it in his power to examine more particularly the nature of corundum.”

Upon comparing the mechanical divisions of the corundum of Ceylon with the Scotch one, we find that it is not only parallel to the six faces of the rhomb, as described both by Bournon and Haüy, but also parallel to eight other faces, all which are mentioned in Haüy’s description of his felspath apyre, two of which are mentioned by him in his Telesie, and the other six not mentioned any where as existing in the corundum of Ceylon, but which we find in some of our specimens. These faces are not so neat, or so easily obtained, as those parallel to the rhomb. The gangue is chiefly composed of a coarse granite intermixed with indurated asbestos.

Mr. Jameson mentions the corundum of Tirie; which however must be very different from this, and he quotes Mr. Greville’s memoirs in Trans. of Royal Society for 1798, page 40, who observes that it scratches glass readily, but not rock crystal. Jameson says, “I believe there are specimens of this corundum in the Museum of the University, and of these I shall probably communicate an account in the close of this volume:” but as he does not seem to say any thing more about it, we hope we shall have it settled in his work now coming out. We preseume that this is no more thought of as a corundum, as C. Bournon in Phil. Trans. 1802 makes no mention of it as such: therefore ours is the only thing known at present as a corundum from Scotland.

  • * Some of ours are also of a greenish tinge, especially when between the eye and the light.
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