468 H. B. PATTON — THOMSONITB, MESOLITE, AND CHABAZITE 



Cleavage flakes and individual leaves taken from both III and Ilia 

 show the same optical properties as do those from II and Ha. 



Mesolite 



This mineral occurs as extraordinarily delicate aggregates, composed 

 of long, slender, hair-like fibers. They are often exquisitely beautiful, 

 and the variety of structure and general appearance appears to be almost 

 infinite. All the specimens thus far obtained may, however, be con- 

 veniently classified under three general types, which are designated re- 

 spectively as types a, b, and c. 



First, type a. The fibers form loosely felted masses that resemble fine 

 cotton wool. 



Second, type b. The fibers are distinct and separate, and form delicate 

 brushes with parallel or nearly parallel bristles. This particular variety 

 is not common. It is apt to grow on the hemispherical bunches of thom- 

 sonite of type Ila. 



Third, type c. The felted aggregate is composed of fibers that lie 

 approximately in one plane, so as to form a fragile gauze or cobweb-like 

 membranes. 



Figures 1 and 2 of plate 46 and figure 1 of plate 47 represent three 

 specimens typical of type a, and figures 1 and 2 of plate 48 exhibit two 

 phases of type c. In figure 2 of plate 47 there is to be seen an excellent 

 example of type b on the right side, while the specimen on the left is 

 intermediate between b and c. 



These three types of mesolite, as well as the numerous intermediate" 

 varieties, may be seen growing on analcite and on thomsonite of all 

 types, but not on chabazite, as this last named zeolite is invariably first 

 covered over by another mineral. They are veiy closely associated with 

 thomsonite of types II, Ila, III, and Ilia, and their extremely deli- 

 cate fibers often appear to be but continuations of the coarser thomsonite 

 growths that support them. More especially is this intimate association 

 to be seen between mesolite and thomsonite of type III. Not infre- 

 quently cavities several inches in diameter (4 to 8 inches) have their 

 centers filled with a rather dense, wool-like, matted aggregate of mesolite* 

 while the outer portion is composed of thomsonite of type III. 



But the determination of this fine fibrous mineral as mesolite depends 

 entirely on the chemical anatysis, as the fibers are far too fine to admit 

 of a decisive physical or optical investigation. Optically about all that 

 can be observed is a parallel extinction, with usually positive extinction. 



It is perhaps well to state that at the time when this paper was pre- 

 pared it was supposed that this mineral, mesolite, was really thomsonite, 

 and specimens sent out from the Colorado School of Mines were so labeled. 



