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Supplementary notes on the mineral Kaolinite

Published online by Cambridge University Press:  14 March 2018

Allan B. Dick
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Extract

In the year 1888 I gave before the Mineralogical Society an account of the forms and optical characters of some minute crystals of kaolinite from Anglesey. At that time I was unable to ascertain whether the mineral was optically positive or negative, and it was stated that the axial angle 2V would approximate to 90°. Subsequently, by the aid of oil-immersion lenses and condensers of high angular aperture (numerical aperture=1.25), it was found that the mineral is optically positive and that the acute bisectrix is normal to the clinopinacoid (the obtuse negative bisectrix emerging through the basal plane). The optic axial angle in air was determined with an eyepiece micrometer to be 2E = 121°, with a possible error of one or two degrees, due to the small size of the crystals and the consequent weak interference-figures.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 15 , Issue 69 , November 1908 , pp. 124 - 127
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1908

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References

Page 124 note 1 Dick, A. B., ‘On kaolinite.’ Min. Mag., 1888, vol. viii, pp. 15–27 CrossRefGoogle Scholar, plate III. Compare Miers, H. A., Min. Mag., 1890, vol. ix, p. 4 Google Scholar.

Page 126 note 1 A. Michel Lévy and A. Lacroix, ‘Les Minéraux des Roches.’ 1888.

Page 127 note 1 Des Cloizeaux, A., ‘Manuel de Minéiralogie.’ 1862, vol. i, pp. 190, 549Google Scholar.

Page 127 note 2 An analysis made on a very small amount of material gave : SiO2, 47.1 ; Al2O3, 39.1 ; H2O, 14.0 = 100.2.

Page 127 note 3 The cleavage flakes usually show a division into three sectors ; in the central one the emergence of the negative bisectrix is normal, whilst in the adjacent sectors it is inclined slightly away from the centre on either side.