260 JPirsson and Robinson — Determination of Minerals 



Art. XXYI. — On the Determination of Minerals in thin 

 Rock-sections by their maximum Birefringence ; by L. V. 

 Pirsson and H. H. Eobinson. 



In the determination of minerals in thin sections of rocks, 

 the common species possess certain diagnostic characters by 

 which they are as a role readily recognized. In doubly refract- 

 ing minerals the interference colors or the birefringence 

 between crossed nicols is probably the most generally useful 

 character for diagnosis and this combined with the refractive 

 index is sufficient in many cases to determine the mineral at a 

 glance, when taken in connection with its color, cleavage, etc. 



In the case of the less common or rare minerals, the approxi- 

 mate determination of the maximum birefringence becomes in 

 many cases of the highest importance and in combination with 

 other characters may be sufficient to definitely determine the 

 species. 



It is thus a matter of the greatest value to the beginner in 

 microscopical petrography that he should learn the values of 

 the maximum birefringence exhibited by the common minerals 

 and associate these in his mind with the colors exhibited 

 between crossed nicols. To do this he should have a simple 

 and relatively accurate method. The more experienced petrog- 

 rapher has of course learned these relations for the common 

 minerals by practice, though generally without any definite 

 method. 



In the case of birefringent minerals the strength of the 

 interference colors shown by sections between crossed nicols 

 depends on three conditions : (1) on the orientation of the sec- 

 tion with regard to the ellipsoid of elasticity, (2) the thickness 

 of the plate and (3) the difference in values between the axes of 

 greatest and of least elasticity. 



If the axial ellipsoid be cut perpendicular to au optic axis, 

 and this in uniaxial crystals is of course a basal section, the 

 section of the ellipsoid has the form of a circle. Hence for 

 parallel light the elasticities are alike in all directions, the plate 

 is isotropic and shows no interference-colors between crossed 

 nicols. As soon, however, as the ellipsoidal cross-section is 

 inclined to this position it takes an elliptical form, the light 

 elasticities are not alike in all directions and hence interference 

 colors are shown between crossed nicols. As the cross-section 

 is inclined more and more the eccentricity of its elliptical form 

 continually increases and with it the strength of the interfer- 

 ence colors until the maximum of both is reached ; in uniaxial 

 crystals this is in the direction of the prismatic zone parallel to 



