Walker — Crystal Symmetry of the Mica Group. 203 



sometimes parallel to, at other times at right angles to, one of 

 the rays of the percussion figure, but never divergent as in the 

 case of biotite and phlogopite. This is all the more remarkable, 

 for the etching figures obtained by both Baumhauer and Wiik 

 are so very plainly triclinic in symmetry. Wiik's figures are 

 reproduced in figs. 4 and 5. 



Muscovite. 

 The optical examination shows no evidence for suspecting 

 that this mica is other than monoclinic, as we at present regard 

 it — this agrees with the etching figures of Leydolt, Baumhauer 

 and Wiik. The optical plane is always at right angles to one 

 of the rays of the percussion figure, as is consistent with 

 monoclinic symmetry. Wiik's etching figures are represented 

 in ii^. 6. 



From the above statement it seems evident that part of the 

 micas are triclinic — biotite, phlogopite, rubellan, lepidomelane 

 lepidolite, and zinnwaldite, while muscovite is either mono- 

 clinic, or, if it be triclinic, it is very finely polysynthetically 

 twinned that we cannot find a triclinic individual large enough 

 to respond to the optical or etching method. According to 

 Mallard, these two are the same — monoclinic crystals (so called) 

 are merely very finely intergrown triclinic twins, the symmetry 

 plane of the monoclinic being due to the increase of symmetry 

 which always accompanies twinning. Levy has suggested that 

 orthoclase or monoclinic potash feldspar is merely a very 

 finely twinned microcline. A similar relation may hold for 

 the micas. 



Some observers have referred to the etching figures on the 

 micas as of monoclinic symmetry. The form of the figures 

 depends on the reagent used, its strength and the duration of 

 the action. Though the physical symmetry of the crystals 

 acted upon is always the same, there may be different amounts 

 of symmetry revealed by different conditions, just as alum 

 may be grown in forms which are geometrically holohedral, as 

 is the case if the crystals are formed in neutral or alkaline 

 saturated solutions, but if grown from hydrochloric acid solu- 

 tions, pyritohedral faces are nearly always present, affirming 

 that all alum crystals are physically hemihedral even when 

 geometrically holohedral. It would be as unsafe to maintain 

 that the micas are monoclinic, because under some conditions 

 certain reagents give etching figures which appear to be mono- 

 clinic in symmetry, as it would be to conclude that all alum 

 crystals are physically holohedral because under certain condi- 

 tions crystals of alum may be obtained which are morphologi- 

 cally holohedral. 



Am. Jour. Sci. — Fourth Series, Vol. VII, No. 39.— March, 1899. 

 14 



