WHERRY AND LARSEN : RHODOCHROSITE AND SIDERITE 367 



this specimen should yield indices almost identical with those of 

 pure rhodochrosite, MnC0 3 . 



The indices of refraction of the Colorado rhodochrosite agree, 

 in fact, closely with the values given by Winchell 4 and e is slightly 

 less than the value given by Ortloff 5 for a specimen containing 

 MnCOs 93.08, FeC0 3 7.12, CaC0 3 0.27, which would be ex- 

 pected to yield a somewhat high result because of the FeC0 3 

 present. The author (E. S. L.) has measured to on several other 

 specimens of rhodochrosite, and found the values almost iden- 

 tical in all of them, as shown in Table 1 . 



TABLE 1 

 Indices of Refraction of Rhodochrosite 



Siderite. Two measurements of the indices of refraction of 

 analyzed specimens of fairly pure siderite are available in the 

 literature. The nearly pure FeC0 3 from Camborne was found 

 by Hutchinson 6 to have w = 1.8724, e = 1.6338, while the sider- 

 ite from Wolfsberg, which is much higher in MnC0 3 and MgC0 3 , 

 is reported by Ortloff 7 to have co = 1.9341, e = 1.6219. These 

 data are inconsistent; the purer specimen should have the higher 

 indices, since the impurities CaC0 3 , MgC0 3 , and MnC0 3 all 

 have considerably lower indices than FeC0 3 . The authors have, 

 therefore, checked the data by measurements on two specimens 

 of recently analyzed siderite. These measurements agree very 

 closely with the data of Hutchinson, so that we regard Ortloff 's 

 value for co as undoubtedly in error, although his value for e is 

 satisfactory. 



4 Winchell,, N. H., and A. N. Optical Mineralogy, p. 149, 1909. 



5 Ortloff, W. Beitrag zur Kenntnis eutropischer Reihen. Zeits. phys. Chem., 

 19: 215. 1896. 



6 Hutchinson, A. The chemical composition and optical characters of chalybite 

 from Cornwall. Min. Mag., 13: 209. 1903. 



7 Ortloff, W. Loc. cit., pp. 215-6. 



