18 



PROCEEDINGS OF THE NATIOlTALr MUSEUM 



VOL. 73 



rhombohedral crystals lining cavities beneath crusts of other zeolites. 

 The larger rhombohedra reach extreme dimensions of 1.5 centimeters. 

 A typical specimen is illustrated in Plate la. The faces of the 

 larger crystals are usually ribbed or striated parallel to the rhom- 

 bohedral edges, the striae meeting in a line which is the shorter 

 diagonal of the face. The chabazite in some specimens rests against 

 the rock of the cavity wall, but more often it has a thin layer of 

 diabantite between it and the wall, and in one specimen a layer of 

 small heulandite crystals lies beneath the chabazite. 



The analyzed sample was prepared from the largest crystal of 

 the lot, which was about 1.5 centimeters on an edge. This was 

 broken up and the best pieces were selected, ground, and sized 

 between 80 and 200 mesh sieves. Under the microscope it was found 

 to be ideally pure, consisting of irregular transparent fragments of 

 very low birefringence, with index of refraction of 1.488 ±0.002. 

 It was biaxial positive with 2V about 30°. The analysis gave the 

 following results : 



Analysis and ratios of chaiazite 



Constituent: Percent Ratios 



s SiO-, 47.66 0.789 0.789 0.197X4 



AI2O3 20.40 .200 .200 .200X1 



CaO 10.52 . ISS] 



MgO .20 .005 ona OOSVI 



K2O .92 .010 -"^"^ .^ObXl 



NaaO .32 .OO5J 



H2O+120°C 16.28 .903\ ,^94 lS2Vfi 



HaO-120° C 3.44 .191/ ^- "^* . iS^Xb 



99.64 



The results agree fairly well with the general formula given by 

 Dana, CaO.Al203.4Si026HoO. The alkalies are rather low, but the 

 mineral is apparently normal chabazite presenting no unusual 

 features. 



EXPLANATION OF PLATES 



Plate 1 



a, Chabazite ; &, mesolite and pseudomesolite ; and c, thomsonite 



Plate 2 



a, Pseudomesolite and analcite, and 6, stilbite 



o 



