Abstract

Study, by means of the Federow universal stage, of the fractures in vein quartz in the Antietam quartzite of Cambrian age in Maryland indicates that rupturing occurs along numerous crystallographic planes. Of these planes the positive and negative rhombohedra are the most abundant, with the prisms m and a, the base c, the pyramids s and ξ well represented.

Identification of the deformation planes was accomplished by first plotting the c axis in the lower hemisphere of an equal net and then rotating it into the center. The poles of all planes were then rotated accordingly and referred to a master sheet containing all the poles of the trigonal trapezohedral or low quartz class.

Fractures are either sharp breaks with or without recrystallized quartz or opaque and fluid inclusions. Some ruptures are not single breaks but are composite, usually consisting of two different planes. Quartz grains tend to break into elongated slivers bounded by crystal planes.

There is a definite relation between the orientation of the quartz grains and the vein wall, but no relation was noted between the rock cleavage and grain orientation. The relation of the fracture pattern to vein wall suggests that both compression and translation of the walls may have produced this pattern and may have caused the rotation of the grains into preferred positions.

Results indicate that quartz is a delicate indicator of stress.

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