Il8 SOMERS: MICROSCOPIC EXAMINATION OF CLAYS 



ters when of larger size, combined with its flaky nature and very 

 often its tendency to combine in fan- or worm-shaped bunches. 

 Low index of refraction and low birefringence separate it from 

 the other micaceous minerals. When fine, it can merely be noted 

 as minute, transparent plates, of an index of refraction close to 

 the balsam, and verv^ low interference color. 



The kaolinite occurs as single scales or plates, sometimes in 

 bunches of fan-shaped character, and as vermiculites. In one 

 clay, the Indianaite from Lawrence County, Indiana, spherulite- 

 like bodies were also found. 



Specially fine examples of "fans" were noted in the clay from 

 Perr\^ Georgia, Bynum, Alabama, Bauxite, Arkansas, and 

 Antioch, California. 



Good examples of vermiculites were seen in samples from 

 South Carolina, Perry, Georgia, Bauxite, Arkansas, Antioch, 

 California, and the nodular white clay from Gordon, Georgia. 



The Florida clay from Okahumpka and Edgar, as well as the 

 samples from Langley and Aiken, South Carolina, showed large 

 single flakes. 



Hydromica. — This is a distinctly micaceous mineral, which has 

 single and double refractions higher than those of kaolinite, yet 

 not so high as muscovite or sericite. Furthermore, the degree 

 of these refractions varies in different clays. It is therefore 

 assumed^ that there is an isomorphous gradation between seri- 

 cite and kaolinite, with a gradual loss of potash and addition of 

 water, and, in weathering products such as these, hydromica 

 represents a transition stage of weathering toward kaolinite 

 as the final product. 



The fan of the hydromica is similar to that of the kaolinite, but 

 it may occur in larger grains. Radiating bunches and spherulite- 

 like grains were found in the white clay from near Huron, In- 

 diana. 



COMPARATIVE ABUNDANCE OF KAOLINITE AND HYDROMICAS 



Descriptions of the microscopic examination of clays that have 



appeared from time to time, make frequent reference to kao- 



^ For discussion of hydromica see Gai,pin, Sydney L. Studies of flint clays and 

 their associates. Trans. Am. Ceram. Soc. 14: 306 and 338. 



