ART. 2. 



PETROLOGY AT GOOSE CREEK SHANNON. 



71 



found most advantageous to measure these in the Goldschmidt posi- 

 tion. The faces, except x (102), are small, and those of a (100), 

 m (110), and c (Tl2) are etched to complete dullness. The tabular 

 face X (102) and n (HI) give fair reflections and m^ (Oil) gives 

 excellent signals. The broad face of x (102) is marked by triangular 

 elevated spots with apices pointing toward a (100) and bases upward 

 toward the position of c (001). The habit of the crystals is indicated 

 in the drawing, figure 22, although in this position the tabular habit 

 of the crystals is not so obvious. 



CHABAZITE. 



Chabazite is one mineral which was 

 found only as drusy coatings along very 

 narrow cracks in unaltered basalt and not 

 in the wider shear zone veins associated 

 with the other minerals. The drusy coat- 

 ings are common and often cover consid- 

 siderable areas, sometimes of a square me- 

 ter or more. The chabazite rs deposited 

 directly on the basalt and is often over- 

 lain by later stilbite and calcite. 



This zeolite is usually in the form of 

 simple rhombohedral crystals of the com- 

 mon unit rhombohedron r as shown in 

 figure 23, or, rarely, interpenetrating 

 twinned rhombohedra. The crystals, 

 which are transparent-colorless to trans- 

 lucent white average only about 1 mm. in 

 diameter. They sometimes are grown 

 into groups in parallel position. 



The comparatively poor crystals which 

 were measured indicated an average 

 rho angle of 52° 48' for the rhombohedron (calculated = 51° 

 25')- There is a moderately perfect cleavage parallel to _ this 

 rhombohedron. 



Optically examined in powder under the polarizing microscope the 

 mineral is found to be zoned parallel to the rhombohedral faces. The 

 central portion of the crystal has the lowest refractive indices, lowest 

 birefringence, and smallest axial angle, the index of refraction, bire- 

 fringence, and axial angle increasing toward the outside. The 

 mineral , thus varies from uniaxial at the center to biaxial with 2V 

 moderate at the peripheries. The indices of refraction vary simi- 

 larlyfroma= 1.485,|3= 1.485,7= 1.490toa= 1.4S8,i3= 1.490,7= 1.495. 

 The optical character is positive. 



Fig. 23.— Chabazite showing unit 

 rhombohedron, the common form 

 AT Goose Creek. 



