176 T. L. Walker— Examination of Trielimc Minerals. 



.Art. XXIV. — Examination of some Triclinia Minerals by 

 means of Etching Figures ; by T. L. Walker, Geological 

 Survey of India. 



(Published by permission of the Director.) 



When crystals are acted upon by corroding reagents the 

 crystal surfaces are seldom dissolved at a uniform rate. Fre- 

 quently they become pitted with small corrosion or etching 

 figures which are bounded by plane surfaces and which indi- 

 cate the symmetry of the crystal surface upon which they 

 occur. If_a holohedral hexagonal crystal bounded by the 

 prism (1010) and the basal pinacoid (0001) be etched, then 

 there should be two kinds of etching figures. As the basal 

 plane has six lines of symmetry, the corrosion figures on it 

 must exhibit six lines of symmetry which must coincide with 

 those of the basal plane. Hexagonal figures whose sides are 

 parallel to the edges (0001) /\ (1010) would satisfy the require- 

 ments of symmetry (fig. 1, p. 181). The figures on the prismatic 

 faces must be different from those on the basal plane. Since 

 each prismatic face is symmetrical in two directions, the cor- 

 rosion pits must also conform to these requirements and must 

 have two lines of symmetry which must be identical in direc- 

 tion with the symmetry lines of the prismatic face on which 

 they occur (fig. 2). If these conditions are not fulfilled, then 

 we are compelled to conclude that the crystal in question is not 

 physically and geometrically holohedral hexagonal but only 

 apparently such. 



As an illustration of this use of etching figures in crystallo- 

 graphic studies we may refer to the case of muscovite, which 

 was supposed to be rhombic until Baumhauer* discovered that 

 the figures produced on (001) by treating with, fused caustic 

 potash, did not show rhombic symmetry. As there appeared 

 to be only one plane of symmetry in the figures while a 

 rhombic basal pinacoid should possess two lines of symmetry, 

 it was concluded that muscovite is only pseudo-rhombic but 

 really monoclinic (fig. 3). 



By employing this method the conclusions arrived at by 

 purely geometrical studies have frequently been corrected. 



Since etching figures ^veal the crystal symmetry, faces 

 which are crystallographically equivalent, such as the eight 

 faces of a holohedral octahedron, the four prismatic faces of 

 tetragonal, rhombic and monoclinic holohedral forms or the 

 two basal planes of holohedral hexagonal crystals, must give 

 similar figures. Parallel faces are generally equivalent but 

 not always. In the case of parallel faces at opposite ends of 



* Sitzber. Akad. Muncheo, 1874, p. 245, and 1879, p. 99. 



