PETROLOGY AT GOOSE CREEK — SHANNON. 



59 



Xi 



in figure 7, which is of a crystal between crossed nicols resting on 

 the pinacoid a (100), the broad flat face. The colors of a typical 

 example are indicated on the drawing. 



This structure is not due to twinning but is due rather to difference 

 in composition resulting in varying birefringence. The variation is 

 probably in the amount of the iron-epidote molecule, this being great- 

 est in the end sectors and gradually decreasing from center to outside 

 in all sectors. The extinction is parallel to the sides of the crystals 

 and the axial plane across the length, this tabular face being nearly 

 perpendicular to the obtuse bisectrix. The crystals vary in color, in 

 ordinary light, fi'om colorless to pale greenish yellow with noticeable 

 pleochroism, the color being distrib- 

 uted in the same pattern as the bire- 

 fringence. This color is visible in the 

 end sectors of the thicker crystals 

 and in the inner tips of the side 

 sectors. 



The epidote is biaxial, negative 

 ( — ), with 2V large. The indices 

 vary with the zoning. One crystal 

 gave, at the outer edges of the side ^ 

 sectors, which is the portion of mini- 

 mum birefringence and probably of 

 minimum index, «= 1.748, j3= 1.754. 



This is typical "hour-glass struc- '^■ 



ture," which is described by Iddings-* 

 as follows: 



Differences in the molecular attractions 2 ^ 



in different directions in a crystal also show 

 "themselves in the constitution of some mixed 

 crystals or crystals of isomorphous com- 

 pounds. It appears as though certain mole- 

 cules in the isomorphous series have a greater tendency to attach themselves in 

 one direction than another ; that is they are more strongly attracted to certain 

 faces of the mixed crystal than to others. The crystal then differs in com- 

 position in segments built up of layers parallel to such faces, which may show 

 themselves in differences of color or refraction. In some minerals the segments 

 are pyramidal with the apexes of the pyramids toward the center of the crj-stal, 

 and the bases at the surface. In sections of such crystals the reversed pyramids 

 sometimes suggest the shape of an hour-glass, hence the term hour-glass struc- 

 ture. The commonest examples of such structure are found in augite in certain 

 basaltic rocks. 



While all of the epidote of this shear zone and that of the specimens 

 collected by Merrill and Wherry are of this peculiar type, the epidote 

 of the miarolitic cavities and that associated with the axinite were 



Fig. 8. —Epidote. Projection of "hour 



GLASS" CRYSTALS ON ft (010) SHOWING 

 OPTICAL ORIENTATION. 



'*J P. Iddings. Rock Minerals, p. 72, New York, 1906. 



