66 



PROCEEDINGS OF THE AMERICAN ACADEMY 



Fig. 8. 

 2i 



I I 



Fig. 9. 



that all micas may be referred to a right rhombic prism with angles of 

 approximately 60^^ and 120'^; secondly, that, while 

 in some micas the plane of the optical axes is 

 parallel to the shorter diagonal of the rhomb, as in 

 Fig. 8, in others it is parallel to the lono-er diasfo- 

 nal, as in Fig. 9. Interpreting these facts by the 

 results of his experiments on isomorphous salts, he 

 draws the inference that there are, crystallograph- 

 ically at least, but two species of mica which are 

 geometrically isomorjjhous but optically distinct, 

 that these are represented by the varieties which 

 have the widest optical angle between the axes in 

 either plane, and that all other varieties with opti- 

 cal angles varying from O'* to 70*^ in either direc- 

 tion are isomorphous mixtures of the two optically 

 distinct conditions of the mineral. 



The observations described in this paper, al- 

 though they prove that another cause may also determine the variation 

 of optical angle in micaceous minerals, do not necessarily invalidate this 

 beautiful theory of Senarmont. The variations observed with other min- 

 erals, not only on different specimens, but with the same specimen at dif- 

 ferent temperatures, and which are so beautiful seen with the orthoclase 

 from Wehr in the Eifel, and with crystals of selenite, indicate that such 

 variations may be determined by conditions of molecular structure 

 wholly independent of the macling here described. We have shown 

 that the macling does jjroduce the variation in certain cases, and it 

 must remain for future investigation to assign the limits of the influence 

 which this cause may exert. We would only remark in conclusion that, 

 although in the 57 varieties of micas examined by Senarmont he did not 

 note a single instance in which the position of the plane of the optical 

 axes — with reference to the diagonal of the rhomb — was different 

 on different parts of the same specimen, or even on different speci- 

 mens from the same locality, he does describe and figure several 

 remarkable macles of muscovite mica similar to those of vermiculite 

 described above and represented by Fig. 5. In one instance the plane 

 of the optical axes is parallel to the shorter, and in the other to the 

 longer diagonal of the rhombic prism ; but in both cases it has the 

 same relative position in the several individuals of the macles, the 

 three planes forming with each other angles of 60^. On the plate of 

 another made a difference of optical angle was observed on different 

 portions of the plate, and this effect was probably similar to that we 



