Wright — Optical Character of Bi refracting Minerals. 285 



Art. XXXI. — The Determination of the Optical Character 

 of Birefracting Minerals y by Feed. Eugene Bright. 



Minerals are recognized in the thin section chiefly by their 

 erystallographic properties and by the effect they have on 

 transmitted light. The more important optic features used in 

 their microscopic discrimination are color, pleochroism, refrac- 

 tive index, birefringence, optical orientation, angle between 

 the optic axes (2V),* and optical character. Of these the latter 

 two are determined in convergent polarized light and are well 

 adapted for general application. They furnish exclusive data 

 as to the nature of a given mineral, and can be accomplished 

 by ordinary petrographic microscopes. 



The optical character of a mineral, whether positive or nega- 

 tive, depends by definition solely on the value of the bisector of 

 the acute angle between the optic axes ; it is, therefore, inde- 

 pendent of the crystal system and pertains to all birefracting 

 minerals. The usual methods available for its determination, 

 however, apply in practice only to uniaxial minerals and to 

 those biaxial minerals for which the angle between the optic 

 axes in air (2E) is less than 80°; if 2E exceeds this limit, the 

 traces of the optic axes lie outside of the microscopic field 

 and give rise to uncertainty as to the position of the acute 

 bisectrix, thereby seriously affecting the results. There are 

 several methods, however, which, although not novel in prin- 

 ciple, are scarcely recognized in literature, and which practi- 

 cally obviate this difficulty. They are based on phenomena 

 observed in convergent polarized light with nicols crossed and 

 apply equally well to uniaxial and biaxial minerals. 



A general consideration of microscopic mineral determina- 

 tion shows conclusively that the optical character of minerals 

 is one of their most useful traits for practical determination 

 since the means employed are simple and of easy application. 

 The following paragraphs aim to present these methods from 

 a working standpoint, the necessary theoretical data appearing 

 in line print. 



The crystal sections of birefracting minerals, from which 

 decisive interference figures can be obtained, are those cut 

 exactly or nearly perpendicular to the bisectrices of the optic 

 axes, to the optic axes, and parallel to the plane of the optic 

 axes. These sections and the methods applicable to them can 

 be discussed for all birefracting substances if uniaxial minerals 

 are treated as a limiting case of biaxial minerals. 



* The rise of the term optic binomial in place of "optic axes" as pro- 

 posed by Mr. L. Fletcher in his treatise on The Optical Indicatrix may be an 

 improvement on the original term, but since the distinction implied by the 

 words uniaxial and biaxial is in rise in all languages, is convenient and 

 causes no confusion, it is probable that the original designation will remain. 



Am. Jour. Sci. — Fourth Series, Yol. XX, No. 118. — October, 1905. 

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