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Author: Admin | 2025-04-28
Figure 15c illustrates crustified gypsum along the walls of an open fracture near the upper contact. The layer of satin spar along the upper contact is continuous and well developed locally. Crystals are orientated parallel to one another (Pl. 19B) and normal to the stratification of the overlying shale, rather than to the upper surface of the gypsum. Most crystals have sharp boundaries and are continuous. Plate 18A illustrates a portion of the bed near the upper contact; optical continuity is maintained between part of the satin spar above the contact and the selenite below. The optical continuity of the selenite is not disturbed by the presence of interfingering shale. Those crystals of satin spar that maintain optical continuity with selenite have indistinct boundaries and exhibit undulatory extinction. Shale, present locally in the satin spar, lies at the end of individual crystals, as illustrated in Figure 15d. Fractures in the satin spar are relatively few and discontinuous. Curvature of satin spar crystals is independent of fracture (Pl. 19A), some crystals maintaining their optical continuity past the line of curvature. The direction and amount of curvature differ in different parts of the satin spar layer. Some satin spar crystals in the overlying shale are curved in a similar manner. Plate 19—Photomicrographs of satin spar. A. Curvature in satin spar, near upper contact of gypsum. Note that some crystal maintain optical continuity past line of curvature. (Crossed nicols, X 42 [adjusted for web]). B. Upper layer of satin spar, separated from underlying gypsum by thin shale larer (dark). Note that satin spar crystals are not oriented norma to upper surface of gypsum (Crossed nicols, X 35 [adjusted for web]). Petrologic Interpretations Original deposition of the gypsum—The gradational zone between the underlying limestone and the gypsum indicates a gradual approach to sulfate saturation of the sea. Inasmuch as large crystals of gypsum showing uniform birefringence and sharp extinction lie along the lower contact, initial precipitation probably was slow, crystallization being concentrated at relatively few points. Calcite, which was still precipitating, accumulated in stringers, which were incorporated within the slowly growing gypsum crystals. As
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