Dihexagonal Alternating Type. 219 



intersections with the crystal face are lines extending 

 parallel to the adjacent edges of the rhombohedron. Two 

 of these figures were produced upon the rhombohedral 

 face, and there was no trace of others. A solution groove 

 developed along the cleavage line of the face, extending 

 parallel to the longer diagonal and extending the entire 

 length. The figures are oriented with the broader end 

 poleward similar to those of smithsonite, but unlike any 

 figures produced by other solvents upon calcite. They 

 are symmetrical with regard to a vertical plane which 

 accords with the symmetry of the face. 



Smithsonite, upon immersion in hot concentrated citric 

 acid, for 15 seconds, acts very much like calcite, and the 

 figures produced are quite similar in shape and posi- 

 tion, — the etchings are so very small due to the size of 

 the crystal face, that no measurements could be made. 

 Thus, while calcite and smithsonite differ considerably 

 when etched by HC1 and HN0 3 , the citric acid figures are 

 quite alike. The unit prism which was so poorly devel- 

 oped on smithsonite failed to etch perceptibly, although it 

 is quite probable figures could be obtained with citric acid 

 if good faces were available. The rhombic carbonates 

 then do differ some in their etching but in general there 

 is a likeness between the calcium, magnesium, iron and 

 manganese minerals of this group. Concerning the rela- 

 tive solubilities of these carbonates it may be added that 

 smithsonite is perhaps most easily attacked, while mag- 

 nesite presents a very marked resistance to the dilute 

 common laboratory acids. 



Base — The base (0001), well developed on the simple 

 crystals of calcite from Patterdale, England, was easily 

 etched by HC1 and citric acids. The HC1 was one-tenth 

 strength and required immersion for fifteen seconds to 

 produce distinct figures (fig. 18) ; these are regular tri- 

 angular pyramidal pits with the three faces lying in the 

 zone of the unit prism, rhombohedron and the base; 

 the bounding lines of the figures are curved, but extend in 

 the general direction of the intersections of the first order 

 prism and base. 



Hot concentrated citric acid if allowed to act for three 

 minutes upon the base, produces well-defined figures, 

 oriented as are the HC1 forms, but the bounding lines are 

 straight, with the three angles rounded, — otherwise they 

 appear identical with the HC1 figures. If, however, the 



