LARSEN AND WHERRY: HALLOYSITE FROM COLORADO 



179 



water, and is less abundant, forming a matrix for nodules of geark- 

 sutite. It is white and opaline to dull in appearance, and has a 

 hardness of about 2. Under the microscope it is isotropic and 

 has a variable index of refraction, averaging about 1.470 ± 0.010 

 when first examined, but increasing to 1.542 after standing for 

 six months in a loosely stoppered bottle in a warm room. 4 



Analysis (by E. T. W.) yielded the results shown in Table 1. 



TABLE 3 

 Change in Optical Properties of Halloysite with Loss op Water 



4 Two halloy sites represented by analyses A and B of U. S. Geol. Survey Bull. 

 591 : 341, were examined microscopically for comparison. That represented by 

 analysis A, from Horse Cove, Hart County, Kentucky, is pale pinkish in color 

 and opaline in appearance. Much of it is weakly birefracting and large areas 

 extinguish much as a unit but with a wavy effect, probably due to strain. The 

 index of refraction is about 1.549 =*= 0.003. The halloysite represented by analy- 

 sis B, from Edwards County, Texas, is chalky in appearance, largely isotropic, 

 and has an index of refraction of 1.556 ="= 0.003, but contains admixed a consid- 

 erable amount of birefracting kaolinite. However, to what extent their indexes 

 have changed since analysis is unknown. 



