12 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 74 



1 part of parasepiolite (A) with 2 parts of paramontmorillonite (B). 

 The isomorphous mixture in these proportions he calls beta-paligor- 

 skite. Under the microscope the Utah material appears in matted 

 aggregated of fine fibers which have a more or less parallel position. 

 The material is not suitable for exact optical measurements. The 

 masses show moderate birefringence, parallel extinction, and posi- 

 tive elongation. The mean index of refraction is about 1.490. The 

 masses of fibers show a confused interference figure with the emer- 

 gence of a negative bisectrix, apparently of large angle and possibly 

 the obtuse bisectrix. This would make the mineral optically positive 

 with Z the elongation. These are similar to the properties given by 

 Larsen for parasepiolite which, however, is opticall}- negative. 



O 



