200 Walker— Crystal Symmetry of the Mica Group. 



man in dieser Weise die Form der Aetzftguren unter alien 

 Umstanden als ausschlagend ansehen, so wurde es hold dahin 

 kommen, dass mannur noch asymmetrische Krystalle 1cennt. v * 

 It may be urged, however, that etching figures are always in 

 agreement with the optical properties, and, so far as we can 

 test them by other reliable methods, they are found to be in 

 agreement. 



Few minerals better show this tendency to degradation from 

 higher to lower symmetry than do the minerals of the mica 

 group. The difficulty in determining the crystal system of 

 the micas by goniometrical measurements is seen in the way in 

 which very eminent minerologists have differed : Marignac and 

 Kenngott held that some micas were hexagonal, and others 

 monoclinic. Senarmont considered them all as rhombic; the 

 late J. D. Dana at one time looked upon them as partly hexa- 

 gonal and partly rhombic ; Ley dolt found the etching figures 

 on muscovite cleavage plates to be symmetrical about only one 

 line, and concluded that the micas were rhombic, but hemi- 

 morphic about the brachy diagonal. In 1875 Tschermak ob- 

 served that the optical axis is not exactly at right angles to the 

 basal cleavage and concluded that the micas were monoclinic. 

 The only further degradation possible for the minerals of this 

 group was suggested by Wiik,f who observed that the etching 

 figures on basal cleavage plates are often asymmetric and con- 

 cluded that at least part of the micas were triclinia These 

 results of Wiik, though well known, have not led mineralogists 

 to look on the micas as triclinic : at present all text-books 

 describe the micas as monoclinic. The object in writing this 

 paper is to emphasize the conclusions of Wiik by giving addi- 

 tional evidence in favor of regarding some, at least, of the 

 micas as triclinic. 



We are accustomed to speak of two classes of micas : 1st, 

 those whose optical plane is parallel to the clinopinacoid, and 

 2nd, those whose optical plane is at right angles to the clino- 

 pinacoid. As the six-rayed percussion figure has one ray 

 parallel to the clinopinacoid, we may determine the crystallo- 

 graphic orientation of crystal fragments by selecting that ray 

 as clinopinacoidal which is either parallel to or at right angles 

 to the optical plane. While examining a specimen of rubellan 

 from the Eifel in this way to determine its orientation, it was 

 observed that the optical plane was neither at right angles nor 

 parallel to any ray of the percussion figure but inclined to one 

 of the rays at an angle of about seven degrees. This led to 

 wider examination of micas and to the following results. 



*R. Brauns, Die optische Anomalien der Krystalle, Leipzig, 1891. 

 fOfvers. Finska Vet. Soc. Forh. 1880. 



