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The identification of dumortierite as grains; dumortierite in Cornish granite

Published online by Cambridge University Press:  14 March 2018

A. W. Groves*
Affiliation:
Imperial College of Science and Technology, London

Extract

The following notes are intended to bring to the notice of workers on heavy mineral grains the possible confusion between blue tourmaline, purple tourmaline, glaucophane, lepidolite, andalusite, pleochroic sillimanite, and dumortierite. Dumortierite (when blue) is very liable to be passed over as the more common mineral glaucophane, the optical properties of the cleavage-flakes of these minerals being similar in several respects.

Having seen a grain of dumortierite from the Wealden of Surrey, kindly lent me by Mr. H. A. Hayward, I was impressed by the remarkable nature of its pleochroism. A day or so later, while examining some concentrates of the Blackheath Beds of Shirley, Surrey, belonging to Mr. G. M. Davies, I came across a single grain which he had described as glaucophane. The pleochroism seemed too intense for a grain of glaucophane of that thickness, and it occurred to me that the tint observed, when the grain was in the position of maximum absorption, was exactly that of the dumortierite grain.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1928

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References

page 489 note 1 Trans. Croydon Nat. Hist. Sci. Soc, 1916, vol. 8, p. 91.

page 491 note 1 Ghosh, P. K., Min. Mag., 1927, vol. 21, p. 296 Google Scholar.

page 492 note 1 Brammall, A., Dartmoor Detritals. Proc. Geol. Assoc. London, 1928, vol.39, pp. 4344 CrossRefGoogle Scholar.

page 492 note 2 Mackie, W., Dumortierite in British rocks. Trans. Edinburgh Geol. Soc, 1925, vol. 11, p. 352 CrossRefGoogle Scholar [Min. Abstr., vol. 3, p. 205]. The paper was read at a meeting of the Society on December 19, 1923, and a report of it appeared in Geol. Mag.. 1924, vol. 61, p. 185.