ART. 16 ZEOLITES PROM OREGON HEWETT, SHANNON, GONYER 15 



filled cavitj'^ about 4 centimeters across. The material is white and 

 pearly in luster, with a fine, confused, platy-massive structure. Em- 

 bedded in the massive material, especially near or attached to the 

 wall, are rosettes 6 to 8 millimeters across composed of radiating 

 blades. The rosettes when crushed give laths lying on a perfect 

 cleavage with two other cleavages bounding the grains. These have 

 elongation Y with the acute bisectrix normal to the perfect cleavage ; 

 are optically positive ( + ) with 2V medium, dispersion r<v strong; 

 indices a= 1.525, ^=^1.527, y=^ 1.530. This is unquestionably thom- 

 sonite like the higher index material of the analyzed sample. The 

 fine, platy-massive material which makes up the solid filling of the 

 cavity shows the same optical characters: Elongation Y, Biaxial 

 positive ( + ), 2V medium, a =1.522, ^=1.525, y= 1.529. This mineral 

 is also thomsonite. 



Stilbite is rather common and forms attractive specimens, some of 

 considerable size, as that illustrated in the lower photograph of 

 Plate 2. In the specimens of this group the only associated mineral 

 is calcite in large, dirty-white cleavage fragments and rhombohedral 

 crystals. These are found both above and below the stilbite crust, 

 indicating two generations of this mineral. The stilbite forms crusts 

 averaging nearly 10 millimeters thick, made up of divergent blades of 

 pearly to silky luster and of white color where unstained. The sur- 

 faces of the crusts have vitreous luster and are made up of large 

 numbers of minute crystal terminations, averaging 0.5 millimeter in 

 size, aggregated in nearly parallel position in rounded groups 5 or 6 

 millimeters across. The principal crystal face is the pinacoid <?(001) 

 of the position of Goldschmidt, modified by sharp and lustrous 

 pyramid faces. The stilbite crusts are more or less stained by iron 

 or soil. The sample for analysis was carefully selected from the 

 cleanest fragments of such a crust. When ground and screened 

 between 80 and 200 mesh sieves and examined under the microscope, 

 it was found to be ideally pure. It was composed of elongate laths 

 lying on the most perfect cleavage and bounded on the sides by a less 

 perfect cleavage. The grains lying on the best cleavage give an 

 interference figure indicating the emergence of the obtuse bisectrix 

 perpendicular to this plane, with negative elongation, and with the 

 optic axial plane parallel with the length. If, as usually considered, 

 the best cleavage is parallel to 5(010) and the elongation of the 

 crystals is vertical, the orientation is X=(?, Y=«, and Z=&. In 

 stilbite Y is usually &, and it therefore seems probable that in the 

 present specimens the orientation is normal crystallographically and 

 optically, but the a cleavage is unusually well developed. All grains 

 show sensibly parallel extinction. The indices of refraction are 



