412 PROCEEDINGS OF THE NATIONAL MUSEUM vol.90 



The indices of refraction, determined by Miss J. J. Glass, are: 

 a = 1.535, /3= 1.539, 7 = 1.544, which correspond to an oligoclase of 

 about Abgo An2o. Such a feldspar would have a composition, accord- 

 ing to the Winchells,^ of SiOa, 63.3; AI2O3, 23.1; CaO, 4.3; NagO, 9.3. 



PYROXENE 



From the chemical composition of the pyroxene shown in table 5, 

 it is evident that the composition is closely related to the hypersthene 

 series. Recalculating the results into their respective molecular com- 

 pounds gives the following percentages: FeSiOs, 31.03; MgSiOs, 58.34; 

 CaSiOa, 9.92; the sum of these is 0.71 percent low, and this may mean 

 that there is a trace of tridymite present in the meteorite. None was 

 noted. 



In the optical analysis made by Miss J. J. Glass, table 6, two slightly 

 dift'erent sets of indices were found. Although the indices for each 

 are very close she has been able to verify the results repeatedly on sub- 

 sequent examinations of additional samples. The physical nature of 

 this material is very uniform, and the two varieties marked A and B 

 in the table can be recognized only by carefully determining their 

 optical properties, as there is no visible difference between the two 

 varieties. 



The material for this portion of the optical examination was obtained 

 from the insoluble residue after the silica had been removed by sodium 

 carbonate as previously stated, but it is believed that such a treatment 

 would not affect a mineral of the enstatite-hypersthene series. Miss 

 Glass reports that all the grains are more or less rounded and seldom 

 show a definite straight edge. 



The extinction angles were measured on what appeared to be 

 cleavage edges, and angles of 6° to 7°, some 11° to 16°, and a few as 

 high as 35° to 40° were found. A very limited number of grains ex- 

 hibited fine lamellar twinning, and an indistinct positive sign was 

 found on two grains. 



By calculation it was found that 28.24 percent of the Chicora 

 meteorite is made up of pyroxenes, and by far the majority of the 

 hypersthene is the variety marked as B in table 6. 



The indices of refraction for the Chicora hyperstene are lower than 

 those given by the Winchells * for a hypersthene with 31 percent of 

 iron metasilicate. The indices found when referred to Winchells' 

 table indicates an iron content of only about 22 percent. The bire- 

 fringence of the Chicora hypersthene agrees closely with the values 

 reported in the Winchell tables. Since the indices of the Chicora are 

 lower than they should be for such an iron content, perhaps the pres- 



'■> Winchell, N. H., and Winchell, A. N., Elements of optical mineralogy, ed. 2, pt. 2, p. 280, 1927. 

 * Op. cit., p. 177. 



