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A possible source of error in the determination of symmetry from optical extinction-angles

Published online by Cambridge University Press:  14 March 2018

Max H. Hey
Max H. Hey*
Affiliation:
Mineral Department of the British Museum of Natural History
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Extract

During an optical examination of many mesolite crystals with a view to a determination of their symmetry, the author was puzzled to find that whereas mesolite has generally been described as anorthic, on optical evidence, he could detect no trace of inclined extinction in the prism zone. A. Cavinato (1926) 1 and G. Ceshro (1909) 3 had already made similar observations. Further study revealed the probable causes of the discrepancies, and as these sources of error, and the precautions necessary for their avoidance, receive little or no mention in the principal text-books, an account of them might be of some use.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 23 , Issue 141 , June 1933 , pp. 367 - 370
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1933

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References

page 367 note 1 Cavinato, A., Atti Soc. Ital. Sci. Nat., 1926, vol. 65, p. 104. [M.A. 4-319.]Google Scholar

page 367 note 2 Cesàro, G., Bull. Acad. Roy. Belg., 1909, p. 435 Google Scholar. The crystallographic .orientation used for mesolite in this note is that proposed by Cesàro.

page 370 note 1 Duparc, L. and Pearce, F., Traité Techn. Min. Pert., Leipzig, 1907, part 1, p. 250 Google Scholar.

page 370 note 2 Vicinal faces, especially if somewhat irregularly developed, may at times cause a crystal laid on a slide to be 1- 2° or more from the supposed position. Cleavage plates may also give false results if the cleavage face on which the plate is lying happens to be stepped to any extent.