some Carbonates of the Calcite Group. 351 



The agreement is rather surprisingly good when the spe- 

 cial nature of the "normal decline' ■ is taken into account. 



The Crystal Structure of Manganese Carbonate (Rho- 



dochrosite). 



The specimens used in this study were slightly pinkish 

 transparent rhombohedrons from Lake County, Colorado. 

 Material from this locality had previously been analyzed 

 and found to be nearly pure manganese carbonate. 30 The 

 value of the axial ratio of MnC0 3 determined from one of 

 these rhombohedrons agreed exactly with that given by 

 Dana. 31 



The size of the unit of structure of rhodochrosite was 

 obtained by making a comparison spectrum of the L-series 

 lines of tungsten from the cleavage, (100), faces of 

 MnC0 3 and CaC0 3 . Taking the value of CaC0 3 as ac- 

 curately known, the spacing for MnC0 3 was obtained by 

 finding first the distance from the crystal to the plate, 

 using the calcite spectrum and then applying this distance 

 to the measurements of the MnC0 3 spectrum. The spec- 

 trum was taken upon a film curved to the arc of a circle of 

 about 10 cm. radius. 



sin 6 = — , 2 in radians == - where 

 2d r 



x = distance of line on film from the central image, and 



r = distance from crystal to film. 



d/n for calcite was taken to be 3.04 X 10' 8 cm. 



From these data and from measurements of x on the 

 film, the ratio of d/n for the (100) face of MnC0 3 was 

 found to be 2.83 X 10 8 cm. This value is considerably 

 different from that already determined. 32 Neither local- 

 ity nor composition was given for the rhodochrosite pre- 

 viously used, so that no explanation of this disagreement 

 can be offered. 



A series of Laue photographs was prepared using a sec- 

 tion cut parallel to the (111) plane. A single photograph 

 was made through a cleavage piece. The patterns ob- 

 tained were similar to those from calcite and required the 

 same treatment in projection and identification of re- 



30 E. T. Wherry and E. S. Larsen, Jour. Wash. Acad. Sci. 7, 365, 1917. 



31 This determination was made by H. E. Merwin of this laboratory. 

 Dana, System of Mineralogy, p. 278, 1892. 



32 W. L. Bragg, op. cit. 



