Becker — Determination of Plagioclase Feldspars. 349 



Art. XLIV. — On the Determination of Plagioclase Feldspars 

 in Rock Sections; by George F. Becker. (With Plate III.) 



In examining the rocks described in a recent report* pains were 

 taken to determine the species of the lime-soda feldspars by 

 modern methods. The difference of refraction between a min- 

 eraland the balsam or between two minerals in contact, accord- 

 ing to the method of Prof. F. Becke, is often useful, and Prof. 

 J. E. Wolff was good enough to determine for me the index 

 of refraction (1*5393) of the Canada balsam used in the final 

 mounting of the slides of the U. S. Geological Survey. Much 

 more useful still is the method which Prof. A. Michel-Levy has 

 developed in two memoirs which should be in the hands of 

 all petrographers.f The method of Mr. Michel-Levy does not 

 necessitate the use of the Fedoroff table, at least in rocks which 

 show an abundance of well-developed feldspars. 



In such rocks it is usually sufficient to deal with the feld- 

 spars, which are cut nearly perpendicular to the brachypinacoid 

 (010), g l or ao P 66. When a crystal is cut in this zone, of 

 course the albite twin lamellae extinguish at equal angles on 

 opposite sides of the cross-hair of the microscope. When a 

 crystal is twinned both according to the albite law and the 

 Carlsbad law, this fact can be detected by placing the trace of 

 the twinning plane at an angle of 45° to the principal sections 

 of the nicols. The associated albite twins are then equally 

 illuminated and cease to be apparent. On the other hand, the 

 Carlsbad twins are not then equally illuminated, so that a crys- 

 tal twinned according to both laws seems to resolve itself into 

 a mere Carlsbad twin. The Carlsbad junction commonly 

 shows signs of interpenetration, and is broken or irregular, 

 while the albite junctions are straight. Supposing such a sec- 

 tion in the zone under discussion, one has in general two sets 

 of albite lamellae, each extinguishing at an equal angle to the 

 cross-hairs, but each pair at a different angle; and furthermore, 

 the orientation of the two pairs of albite lamellae bear a defi- 

 nite relation to each other, because the difference of orientation 

 is due to Carlsbad twinning. If angles in the zone are counted 

 from the front edge of the prism, and if one pair of albite 

 lamellae is cut by a plane making an angle (/> with the front 

 edge, then the other pair of lamellae is cut at an angle of 

 180°- to the same edge. Mr. Michel-Levy's beautiful stereo- 

 graphic projections (first fasicle) show how the extinctions in 



* This note is an extract from a paper on the Gold fields of Southern Alaska, in 

 the Ann. Rep. U. S. Geol. Surv , 1896-97. part III. 



f £tude sur la determination des feldspaths, 1894. Same, second fascicle, L896. 





