366 WHERRY AND LARSEN : RHODOCHROSITE AND SIDERITE 



content as 3.62 per cent, and the specific gravity 3.69, on the au- 

 thority of J. B. Mackintosh. Prof. E. S. Dana states 3 that the 

 angle r A r' is 73° 4^'. In the course of the determination o c the 

 optical constants of minerals one of us (E. S. L.) desired to ob- 

 tain the indices of refraction of the purest possible rhodochrosite 

 and in order to confirm the composition of the mineral in this 

 occurrence, a specimen (No. 86249) in the National Museum 

 collection was analyzed (by E. T Wherry). The results of the 

 analysis were: MnC0 3 95.72, FeC0 3 1.87, CaC0 3 0.50, MgC0 3 

 0.68, gangue 0.82 per cent, sum 99.59 per cent. This represents 

 a lower percentage of iron than previously reported, but un- 

 doubtedly the relative amounts of isomorphous constituents vary 

 from one specimen to another. That this specimen is actually 

 somewhat different from the previously described one is shown 

 by the angle r A r', which, measured on cleavage planes, varies 

 from 73° 10' to 73° 20', always exceeding the earlier measure- 

 ment, and the specific gravity, which is 3.71, distinctly higher 

 than that above quoted. 



The indices of refraction were measured by E. S. Larsen by 

 the immersion method. The index e was measured directly, and 

 checked by measuring the highest and the lowest indices on the 

 very perfect cleavage pieces and computing the value of e from 

 the formula for the indicatrix 



x* if- 



- + J - = 1 



or e 



knowing the cleavage angle and, hence, the angle of inclination 

 of the section with the basal plane. The results were: a> = 

 1.817 ± 0.003, e = 1.595 ± 0.005, measured directly; e' = 1.699 

 ± 0.003 on the cleavage piece, hence, e = 1.594 ± 0.005. 



The indices of refraction of the rhombohedral carbonates 

 are greatly affected by isomorphous replacements, but as in this 

 case the total replacement is only about 3 per cent and as the 

 calcium and magnesium carbonates, which lower the indices, 

 are in such proportions as to compensate approximately the effect 

 of the iron carbonate, which increases the indices (see below), 



3 Dana, E. S. System of Mineralogy, 6th ed., p. 279, 1892. 



