AET. 16 ZEOLITES FKOM OREGON HEWETT, SHANNON, GONYER 17 



ANALCITH 



Analcite forms normal trapezohedral crystals up to 1 centimeter in 

 diameter in a number of specimens, invariably in association with 

 pseudomesolite. The two minerals seem to have grown almost simul- 

 taneously, although the analcite is slightly later in age. One of 

 the best specimens is shown in natural size in the upper illustration 

 of Plate 2. In this specimen the mode of occurrence in an irregular 

 cavity in highly vesicular lava can be seen to advantage. The asso- 

 ciated acicular crystals and fibrous material are pseudomesolite, 

 Sometimes these two minerals rest upon the basalt itself but in most 

 cases they are preceded by a thin sldn of chabazite. In the hand 

 specimen the analcite crystals range from grayish and translucent 

 to vi^hite and opaque, the difference depending on the quantity of 

 included pseudomesolite. 



The sample for analysis was prepared by cracking up selected 

 large crystals, of which the cleanest and most glassy-translucent 

 fragments were picked out, ground, screened between 80 and 200 

 mesh sieves, and purified as far as possible with heavy solutions. 



Under the microscope the analyzed sample was composed of clear 

 grains which showed no cleavage, were uniformly birefringent in 

 low-order gray with grating structure, and give a biaxial negative 

 ( — ) figure with 2V small and dispersion weak, r<v. The mean 

 index of refraction is 1.488. All of the grains show a few penetrating 

 fibers of pseudomesolite but the amount of this is not sufiicient to 

 affect the analytical results seriously. Upon analysis this sample 

 gave the following results and ratios : 



Analysis and ratios of analcite 



Constituent : Per cent Ratios 



Si02 53.92 0.893 0.893 0.223X4 



AI2O3 24.69 .243 .243 .243X1 



CaO .96 .0171 



MgO .04 .001 204 204X1 



K2O 2.24 .0241 -^^^ .^U4X1 



NaaO 10.04 . I62J 



HzO + llO" C 8.62 .479 .479 .239X2 



H2O-110° C None. 



100. 51 



These results approximate as closely as usual the formula 



Na2O.Al2O3.4SiO2.2H2O 



which is the formula given by Dana. The material is thus normal 

 analcite and presents no unusual features. 



CHABAZITE 



Chabazite appears in numerous specimens, but in very few is it 

 conspicuous. Its most common mode of occurrence is as tiny white 



