292 SENARMONT ON THE OPTICAL CHARACTERS OP 



of the optical axes varies from 49° (ia the yellow Brazilian 

 topaz) to 65° (in the blue crystals from Aberdeenshire), and the 

 micas, in which the deviation commences at 0° and rises as high 

 as 45°, and even more*. 



All these anomalies present themselves likewise in the mixed 

 tartrates, for the angle formed by the optical axes may amount to 

 as much as 75°, pass through 0°, and even open to the extent of 

 60° in a plane normal to the original plane. I have indeed artifi- 

 cially prepared crystals, which are in every respect analogous to the 

 micas, which are shown by their optical characters and chemical 

 relations to result from the crystalline association of several iso- 

 morphous compounds. These experiments are in reality nothing 

 more than synthetic demonstrations of the causes which are 

 capable of giving rise to the formation of minerals chemically 

 and optically different ; and they justify us in entertaining the 

 opinion, that the family of micas will, when better studied, be 

 found to comprise varieties in which the plane of the optical 

 axes is situated in two diametric rectangular planes f. 



Chemical analysis has not as yet thrown so much light upon 

 the composition of the varieties of topaz as it has upon that of 



* The form of the micas can never be determined with accuracy, and the re- 

 sults of their chemical analysis scarcely admit of rational interpretation. It is 

 therefore possible to doubt the existence of two species in right and oblique 

 rhombic prisms. 



These prisms would both have an angle of 120°, and would both be cleave- 

 able in a direction parallel to their respective base ; but in the oblique form 

 there is no crystallographic symmetry which renders it necessary that the op- 

 tical axes for different colours should have but one bisector, and that this should 

 be normal to the base. On the contrary, such a singular arrangement would 

 be purely fortuitous, and even exceptional, while in the rectangular form it is 

 necessary. But it does not appear that in this respect any essential difference 

 has been observed in the several varieties of mica ; and this circumstance is 

 alone sufficient to throw considerable doubt upon the division adopted by many 

 mineralogists. 



•}• M. Biot has always found the plane of the optical axes in the micas to be 

 parallel to the minor diagonal of the rhombic base {Mem. de I'Institut, 1816, 

 p. 275) ; and at the time when this memoir was written, I was not acquainted 

 with any other complete series of optical experiments on these minerals. Since 

 then 1 have met with a very extended optical investigation by Mr. Silliman, 

 Jun., in which he states that in some varieties, to which Mr. Dana has applied 

 the name of Muscovite, the plane of the optical axes is parallel to the major 

 diagonal. In muscovite the optical axes generally present a very large angle 

 (American Journal of Science, 2nd series, vol. v.). 



