Allanite in the granite of the north of Scotland / by William Mackie.

  • Mackie, William
Date:
[1908]
Catalogue details

Licence: In copyright

Credit: Allanite in the granite of the north of Scotland / by William Mackie. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    or two slides in relatively small crystals, and these in the cases observed have been cut transversely. In Ben Nevis granite one small zoned crystal has been observed, and that was in the most acid variety of the Ben Nevis rock which contains no horn- blende and very little biotite. In the red granite of Peterhead it has been observed twice—in one case in a beautifully striated, rather narrow prism associated with biotite. The other was a transverse and somewhat opaque section in which the characters of the mineral were observed with difficulty. Magnetite existed as an inclusion in the former. In a highly acid biotite granite showing strong cataclastic features from Loch Hoag in Lewis, and of which a red and a grey variety occur, specimens of which were kindly sent me by Mr T. Fraser, C.E., Inverness, allanite frequently occurs in small prisms and always where observed in association with epidote. Extinction angles of 40° and 36° with the long axis of the prism have been noted. Here magnetite has been repeatedly observed as an inclusion. A single prism has been observed in a granite from Kennethmont, which is associated with the diorite of Aberdeenshire, and shows a large amount of biotite and a very little hornblende in addition to the ordinary constituents. This example gave an extinction angle of only 24° with the long axis of the prism, but that was probably cut more or less transversely. It showed beautiful zoning as well as a mottled green and brown centre. In a specimen of the granophyre of Skye from a gully on the N.W. of Marscow, sent me by Dr Horne, what is evidently allanite occirrs associated with green augite, and a little hornblende, in a coarsely micro- pegmatitic rock which shows idiomorphic quartz crystals set in orthoclase. It is in somewhat irregular grains which show irregular Assuring, and has partly decomposed, staining the surrounding section a brown colour. A similar staining, it should have been stated, surrounded the crystal in Peterhead granite. Extinction angles, in the Skye specimen of 38°, 26°, 36°, were observed in different crystals. In the Assouan granite I the allanite was included in orthoclase and gave an extinction angle of 40°. The peculiarities of allanite as regards distribution and occur- rence appear to be—it is independent of the character of the rock ias regards acidity or basicity; it is very frequently associated 'with epidote ; often contains magnetite as an inclusion,—indeed Lhat is up to the present the only inclusion observed ; it always appears early—if not in fact the very first in the order of ■crystallization. This latter characteristic is the more remark- able, seeing that it is one of those hydrated minerals which as a class are unstable at high temperatures. Its occurrence must, therefore, indicate conditions of enormous pressure at VOL. IX. PART III. 1>
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