lioness — Etching Figures. 201 



Art. XIV. — On the Etching Figures of the Dihexagonal 

 Alternating Type; by Arthur P. Honess. 



Introduction. 



While the dihexagonal alternating crystal type includes 

 a variety of mineral species, calcite, due to its abundance 

 of forms, is perhaps more representative than any other. 

 For this reason the etchings of calcite form the basis of 

 this investigation; and, although some work has been 

 done on the rhombohedrons of this species, by other 

 authors, an extended study of calcite, with a view to the 

 correlation of the results arrived at geometrically and 

 those obtained by the etch method, has not hitherto been 

 made. Consequently, a more detailed examination of the 

 etchings of this important mineral, and their relation to 

 the symmetry content, may be of some interest. 



Tschermak 1 in his article, "Ueber die Isomorphic der 

 rhomboedrischen Carbonates und des Natrium-salpe- 

 ters," gives the result of his investigation of several of 

 the more common isomorphous carbonates. 



The results upon calcite are in perfect accordance 

 with what is expected, — HC1 produced curved triangular 

 forms, with the point turned upward, revealing a ver- 

 tical plane of symmetry. With H 2 S0 4 he obtained 

 rhombic forms identical in form and position with the 

 rhombohedral face itself, also symmetrical with regard to 

 a vertical plane. 



For the purpose of comparison Tschermak etched mag- 

 nesite, siderite, rhodochrosite and sodium nitrate, but 

 was unable to secure suitable smithsonite crystals for 

 this purpose. 



Magnesite was etched with HC1, and very dilute 

 HoS0 4 , two kinds of figures resulting. Most of them 

 were simple symmetrical triangular forms, but a few 

 asymmetric figures were noticed. Thinking the iron 

 content of his Zillerthal crystals might be the cause of 

 this variation, he obtained pure magnesite from Snarum, 

 and etched this, only to obtain similar results ; explained 

 by the fact that the crystals were mixed: in part rhom- 

 bohedral, in part tetartohedral. 



1 Min. petr. Mitt., 4, 99, 1881. 



Am. Jour. So.— Fourth Series, Vol. XLV, No. 267. — March, 1918. 

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