Walker — Crystal Symmetry of the Mica Group. 199 



Art. XX. — The Crystal Symmetry of the Minerals of the 

 Mica Group ; by T. L. Walker, Assistant Superintendent 

 Geological Survey of India. 



From the time that the symmetry of crystals was deter- 

 mined exclusively by the aid of goniometrical measurements, 

 there has been a constant degradation of crystals from systems 

 of higher symmetry to those of lower symmetry. Many sub- 

 stances which were once regarded as tetragonal are now looked 

 upon as rhombic ; others that were supposed to be rhombic 

 have been shown to be monoclinic, while monoclinic has fre- 

 quently given way to triclinic. The movement has always 

 been downward, lower symmetry replacing higher. This has 

 been due in a small measure, to the improvement that has been 

 made in goniometers, but principally to the progress of physi- 

 cal investigation of minerals. At first it was considered 

 remarkable that the optical properties of cubic crystals should 

 be strikingly different from those of tetragonal and hexagonal 

 crystals, which in turn differed widely optically from rhombic, 

 monoclinic and triclinic crystals. At the present time, however, 

 physical crystallography is looked upon as so important that 

 no crystal can be admitted to any crystal system till it has 

 been examined as to its optical, thermal, electric and cohesive, 

 as well as to its geometrical properties. Only when the physi- 

 cal and morphological properties agree in every respect, is it 

 safe to assign a crystal to a particular system. 



The degradation caused by placing physical and morpholog- 

 ical examination of crystals on equal footing, found its climax 

 in the writings of Mallard,* who seems to have concluded that 

 all crystals are composed of asymmetric particles which may 

 by twinning simulate higher grades of symmetry — sometimes 

 we can detect this twinning geometrically or physically, but 

 even when we cannot detect it, we are to suppose it to exist 

 on a scale so fine that our means of examination are insuffi- 

 cient. 



The study of etching figures as an indicator of crystal sym- 

 metry, is one of the newest means discovered by the physical 

 crystallographer. This method is often so much more delicate 

 than even the optical method that it points to wholesale degra- 

 dation. Not only is it much more delicate than the optical 

 method, but it can often be used when no conclusion can be 

 obtained by any other means. There have been many protests 

 against admitting the evidence of this latest and most radical 

 witness, as is exemplified by Brauns, when he says : " Wurde 



* E. Mallard, Explication des phenomenes optiques anomaux, Paris, 1877. 



