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Fibrolite (= Sillimanite) as a gem-stone from Burma and Ceylon1

Published online by Cambridge University Press:  14 March 2018

L. J. Spencer
L. J. Spencer*
Affiliation:
Mineral Department of the British Museum
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Extract

The only book on precious stones in which fibrolite is taken into account appears to be that of .J. Escard, ‘Les pierres précieuses’ (Paris, 1914, p. 190). He mentions the prehistoric stone implements made of dense fibrolite tbund in France, ~nd also states that the mineral is sometimes used as an ornameutal stone. Examples of such material are represented in ttm British :Museum collection of minerals by: a hatchet, measuring 11×½ cm., of pale yellowish-grey, compact fibrolite from N.W. France ; and a polished knife-handle, 9 cm. long, of greyish-white, transluceut, fibrous material with silky lustre. Clear material of good colour and cut in the form of a faceted gem has not hitherto been described.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 19 , Issue 91 , December 1920 , pp. 107 - 112
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1920

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Footnotes

1

Communicated by permission of the Trustees of the British Museum.

References

page 107 note 2 There appears to be no good reason for rejecting Bournon's earlier (1802) name fibrolito in favour of the name sillimanite proposed by G. T. Bowen in 1824. J. D. Dana in the fifth edition (1868) of his 'System of Mineralogy' adopted the name fibrolite.

page 107 note 3 Grünling, F., Zeits. Kryst. Min., 1900, vol. 88, p. 286 Google Scholar, and described, with detailed determinations of the optical constants, by G. Melezer, ibid., p. 258.

page 107 note 4 Laeroix, A., Bull. Soc. franç. Min., 1889, vol. 12, p. 29 Google Scholar.

page 107 note 5 Coomaraswamy, A. K., Spolia Zeylaniea, 1904, vol. 2, p. 60 Google Scholar ; 1905, vol. 3, p. 59 (here the remark ‘Very rarely in good crystals in gem gravels’).

page 108 note 1 Ceylon, Report on the results of the Mineral Survey in 1905-6, Colonial Reports, Miscellaneous No. 42, London, 1907 ; ditto for 1906-7 and 1907-8, ibid, No. 74, 1910 (see, e.g., p. 85) ; ibid., No. 87, 1914.

page 108 note 2 Brown, C. B. and Judd, J. W., Phil. Trans. Roy. See. London, 1896, ser. A, vol. 187, pp. 195, 213Google Scholar.

page 109 note 1 The value (H.=6-7) given in the textbooks is too low, due, no doubt, to determinations made on aggregates of crystals rather than on a single crystal.

page 109 note 2 Previous determinations of the optical constants of fibrolite have been tabulated by O. Melczer (1900, loc. cit.). Later determinations are by E. Taubert (1905) and, on artificial material, by W. Eitel (1914) and by G. A. Rankin and F. E. Wright (1915). These show a greater range of variation than would be expected in a substance of simple composition (Al2O3.SiO2) and without isomorphous replacements.

page 109 note 3 This was done with the small and handy form of crystal-grinding apparatus designed by H. H. Thomas and W. Campbell Smith, this Magazine, 1914, vol. 17, p. 86. When grinding a surface peq)endicular to a perfect cleavage, it is a good plan to slightly bevel the edges to prevent fraying.

page 110 note 1 In a properly proportioned faceted gem this should, of course, not be possible, since all light entering the stone by the table should be totally reflecr inside by the back facets.

page 111 note 1 E. Taubert (Das Aehsenverhätltnis des Sillimanit. Contralblatt Min., 1906, p. 872) gives the axial ratios a : b : c : = 0.9696 : 1 : 0.7046, the value for c being given by a single terminal face q (052) observed on a small crystal from Chester, Connecticut. Referred to the axial ratio given above this form becomes (043) : calculated to b 29° 18', measured by Taubert 29 ~ 85'. Some early measurements were given by G. T. Bowon (1824) and J. D. Dana (System of Mineralogy, 2nd ed., 1844), but these were later rejected by Dana as being too approximate.

page 112 note 1 The same term would also apply to the finely fibrous, translucent mineral (as in the knife-handle mentioned above), but in that case the optical effect is due to the aggregation of fibrous crystals, and not to the inclusion of fibres (or lines) in a single crystal.