372 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



Comparison of optical properties of sericite and muscovite 



(l) 

 Sericite (Ross) 



Biaxial. 

 Negative ( — ). 

 2V=25°. 

 X=c. 

 Indices: 



«=1.550±0.003. 



0=1.585 .003. 



7 = 1.587 .003. 

 Birefringence 0.037. 



(2) 

 Muscovite (Larsen) 



Biaxial. 

 Negative ( — ). 

 2V = 40°. 

 X = c. 

 Indices: 



a= 1.561. 



0=1.590. 



7 = 1.594. 

 Birefringence 0.033. 



While the indices are slightly lower and the birefringence a little 

 higher, the optical properties are not greatly different from those of 

 ordinary muscovite. 



Heated before the blowpipe the sericite fuses on the edges to a 

 white enamel. This property is important since muscovite and 

 kaolinite are commonly listed as infusible minerals and leverrierite 

 has heretofore been separated from these minerals on the basis of its 

 fusibility. Heated in the closed tube the mineral first darkens and 

 then becomes white again and yields water having an acid reaction. 



Optical examination of the material which had been selected for 

 analysis showed it to be essentially homogeneous except for the 

 presence of two minerals in small amount, one certainly colorless 

 zircon in minute crystals and the other an isotropic mineral in colorless 

 grains of high refractive index, quite probably the light-colored 

 sphalerite mentioned above. These were entirely removed by a 

 method of selective crushing and screening. A screen was selected 

 with meshes smaller than a majority of the sericite flakes and the 

 material was screened repeatedly, alternating the screening with 

 rolling the powder, spread thinly on plate glass, with a hardened 

 steel rod. The brittle impurities were thus crushed so as to pass 

 the screen while the micaceous sericite remained behind. Even 

 before purification the impurities amounted to only a fraction of 1 

 per cent. Upon analysis this purified sample gave the following 

 results and ratios: 



Analysis and ratios of sericite from Carroll-Driscoll mine 

 (E. V. Shannon, analyst) 



Silica (S1O2) 



Alumina (AI2O3) 



Ferric iron (Fe203) 



Lime (CaO 



Magnesia (MgO) 



Potash (K2O) 



Soda (Na 2 0) __._ 



Water (H2O) above 110° C 

 Water (H2O) below 110° C 



Total 



.443X1 1.15X1 



