A petrographic and chemical study of ternary feldspars



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Luna Vista sill Is a recently exposed, irregular, elongate Intrusive body consisting of a trachytic, sparsely porphyritic sodium-rich quartz alkali microsyenite. The sill is broadly basin shaped and is slightly discordant with underlying sediments of Late Cretaceous age. Alkali feldspar phenocrysts and felsic groundmass account for about 84 percent of the rock. The sill was originally more laterally extensive, especially toward the northeast and northwest where sapping caused by the underlying claystone has brought about extensive retreat of the cliffed margin. Feldspar phenocrysts and groundmass crystals from the sill were analyzed with the electron microprobe and were determined to have a narrow compositional range of zoning which follows trends indicated by phase equilibrium studies of feldspar systems. Two synthetic gel mixtures that were similar in composition to the sill rock feldspar composition and a finely powdered portion of the sill rock were melted and crystallized experimentally at linear cooling rates at 5, 2, and 1 kb. Feldspar crystals grown in these experiments were also zoned according to predicted trends. There are five different zoning patterns found in the crystallized gel samples which are mainly controlled by the rate at which crystallization components arrive at the growing surface. The phenocrysts of the natural rock are believed to have crystallized at much slower cooling rates and stabilized at conditions closer to an equilibrium state than did the crystals formed in the experimental runs, where considerable departure from equilibrium conditions prevailed. This prevented extensive comparison of crystals from the two sets of conditions.



Feldspar, Texas, Big Bend Region, Sills (Geology)