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CHAMBERED CALCAREOUS SPONGE: Up. Permian (Guadalupian) Cherry Canyon Fm., Culberson Co., Texas. This example shows a chambered calcareous sponge that was partially replaced by silica, and the central cavity was filled with multiple generations of chalcedonic cement. Radiating splays or fans of fibrous crystals, low birefringence and pseudo-uniaxial crosses all are characteristic features of chalcedony. The color banding, visible even in cross-polarized light, is due to variations in concentration of minute aqueous (and possibly also mineral) inclusions. Chapter 28: CARBONATE DIAGENESIS: Silica cementation and replacement; p. 410. Memoir 77: A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis by Peter A. Scholle and Dana S. Ulmer-Scholle
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CHAMBERED CALCAREOUS SPONGE: Up. Permian (Guadalupian) Cherry Canyon Fm., Culberson Co., Texas. This example shows a chambered calcareous sponge that was partially replaced by silica, and the central cavity was filled with multiple generations of chalcedonic cement. Radiating splays or fans of fibrous crystals, low birefringence and pseudo-uniaxial crosses all are characteristic features of chalcedony. The color banding, visible even in cross-polarized light, is due to variations in concentration of minute aqueous (and possibly also mineral) inclusions. Chapter 28: CARBONATE DIAGENESIS: Silica cementation and replacement; p. 410. Memoir 77: A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis by Peter A. Scholle and Dana S. Ulmer-Scholle