Dihexagonal Alternating Type. 217 



acted very much like the calcite when in contact with the 

 acid, and upon examination revealed figures very similar 

 to the calcite figures (fig 15) ; they are triangular and the 

 majority are bounded by the three faces intersecting in a 

 manner very much like calcite and siderite ; the shorter 

 side or face, however, dips more abruptly than does the 

 calcite form. Occasionally there is a basal plane present, 

 but this is usually small and rare. 



Increase in time from 20 to 30 seconds' immersion does 

 not change the shape of the figure to any great degree, 

 but merely enlarges it. Like the other carbonates the 

 face is well cut with solution grooves along the cleavage 

 lines. The figures are symmetrical to a vertical plane 

 cutting the rhombohedral face. 



The small transparent crystals of smithsonite were 

 exceedingly soluble, even more so than the calcite, conse- 

 quently immersion for two seconds was sufficient to etch 

 completely the well-formed rhombohedrons and the 

 poorly developed unit prism 1010. Examination revealed 

 a striking resemblance between the unit prism figures of 

 calcite and those of the smithsonite, and still a more strik- 

 ing contrast in the rhombohedral figures of the two min- 

 erals. The prism being very small with a very imperfect 

 surface could not be etched so satisfactorily, but the few 

 figures studied seem to be almost identical in form and 

 orientation with those of calcite. The figures of the 

 rhombohedral faces are very unique, unlike any other fig- 

 ure produced throughout the entire investigation; they 

 are very angular and composed of but two triangular 

 planes, intersecting to form the basal groove which 

 extends parallel to the short diagonal of the rhombohe- 

 dral face. The apex was turned upward and in most 

 cases was approximately 90° ; some were less. The deep- 

 est portion of the figure appeared to be at this point, with 

 the two triangular faces gradually ascending from it. 

 All boundaries are straight and with very sharp angles. 

 The figures are symmetrical to a vertical plane. • 



On the whole, then, there is not found to be the resem- 

 blance which might be expected in the etch figures of the 

 rhombohedrons of the isomorphous series of carbonates, 

 even when the conditions of etching were about the same 

 for all minerals. If there is a resemblance to be noted it 

 must involve calcite, magnesite and rhodochrosite ; and 

 yet, while siderite is in some ways distinctly different. 



Am. Joue. Sci.— Fourth Series, Vol. XLV, No. 267.— March, 1918. 



15 



