I20 SOMERS: MICROSCOPIC EXAMINATION OF CLAYS 



The identification of kaolinite rests on the following evidence: 



(a) The index of birefringence is distinctly low, about that 

 of quartz, but variable. 



(b) The index of refraction is very near to i .56. 



(c) The prismatic crystals extinguish parallel to the basal 

 plane. 



(d) Basal flakes show a biaxial interference figure. 



While Hickling refers to the mica as muscovite, he notes that 

 it may be hydrated ; indeed he thinks that the muscovite changes 

 directly to kaoHnite in the clay, because: i. He can find mica 

 but no kaolinite in feldspar or in the granite. 2. There is no 

 difference in form between the mica and kaolinite. 3. He finds 

 prisms which are mica at one end and kaolinite at the other. 



He also quotes the observations of Johnstone, ^ who found that 

 by exposing muscovite to pure water, and Vv^ater saturated with 

 CO2, for 12 months, it had changed to hydromuscovite. This 

 Hickling believes shows a conversion in the direction of kaolinite. 



In line with these observations, reference may be made to 

 the work of Galpin^ on flint clays. In these he found platy masses 

 of what at first appeared to be kaolinite, which frequently show 

 "ribs" or plates of higher index and birefringence intergrown 

 with those of kaolinite, and which ribs show practically every 

 grade of variation between kaolinite and muscovite. 



Halloysite. — Two clays, viz, those from north of Huron, 

 Indiana, and the Bond-Marks deposit near Beatty, Nevada, 

 contain material in platelike grains, which is isotropic, and is 

 probably to be regarded as halloysite. In no other samples 

 could this material be so definitely identified. 



Rutile. — The presence of rutile is interesting, in view of the 

 probable constant occurrence of titanium in high-grade clavs. 

 Practically every clay examined shows rutile in some amount. In 

 some cases it is in grains or prisms, perhaps 0.015-0.020 mm. in 

 diameter, when its color and refractive properties distinguish 



■* Johnstone, A. On the action of pure -water and of -water saturated -with carbonic 

 acid gas on minerals of the mica family. Quart. Journ. Geol. Soc. 45: 363. 1889. 



* Galpin, S. L. Studies of flint clays and their associates. Trans. Am. Ceram. 

 vSoc. 14: 301. 1912. 



