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I.—On the Microscopic Structure of Luxullianité

Published online by Cambridge University Press:  14 March 2018

T. G. Bonney
T. G. Bonney*
Affiliation:
St. John's College, Cambridge
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Extract

So far as I am aware no account of the microscopic structure of this rare and beautiful rock has yet been published. I have, therefore, thought that a brief description of its structure and some considerations concerning the origin of tourmaline rocks may be of interest to petrologists.

Boulders of the rock, as is well known, are abundant in the vicinity of the village of Luxullian (about five miles from the town of St. Austell, Cornwall), where I collected specimens in tho autumn of 1873; but I believe the rock itself has never been discovered in situ, though veins of different varieties of tourmaline rock are abundant in the granite of this and other districts in Cornwall, some of which occasionally rather resemble it, and the mineral frequently occurs in the granite itself, the felsite cleans, and the altered sedimentary rocks.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 1 , Issue 7 , November 1877 , pp. 215 - 221
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1877

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References

page 215 note * It will not be forgotten that a magnificent block of this rock is used for the sarooplmgns of the late Duke of Wellington.

page 216 note * For brevity I will employ this name to designate the bluish variety. This would be a more distinct black when thick than the other,which inclines to yellowish brown. As will be seen hereafter, I suspect there is a difference in the chemical composition of the two kinds.

page 217 note * We have not attemptmd tm represent this dichroism in the figure.

page 219 note * In another slide from the Mousebole rock the schorl generally borders the brown tourmaline where it is contiguous to felspar crystals.

page 219 note † I am aware that the opinions above expressed differ from those of Dr. Senft (Die Krystalliaische Felsengemengtheile, &c., p. 505), who regards mica as an alteration product of tourmaline, and gives a section which appears to favour his view. Not having seen the locality, I cannot say whether his interpretation is tile only one possible ; and at any rate it leaves quite unanswered the difficulty of what fired formed the tourmaline rock from either granite or quartz (the rock adjoining the veins) ; we must also remember that the order of chemical change in nature not seldom differs. Be this, however, as it may, I have carefully considered the question and can put no other interpretation than the above on the microscopic sections which I am now describing.

page 220 note * In like way I have seen acicular crystals of secomiary actinolite pierce into crystals of comparatively undecomposed felspar. This is described in a paper presented last May to the Geological Society, but not yet published.

page 221 note * It must also be remembered that water is a constituent of tourmaline, which favours the idea of its being a secondary product:

page 221 note † Sir H. De la Beche op: cir. p. 160, regards the sehorl as, at any rate in many cases, a secondary product, though he does not appear to have considered the details of the question. Since the above was written my attention has been eal]ed to a notice in the Comptes Rendus for 1864, by M. Pisani (p. 913). He regards Luxullianite as an altered granite and states that tourmaline has replaced the mica; the opinion, however, seems to be a conjecture formed on a general examination without the microscope, as the rock is said to contain very little quartz, which is hardly correct.