Physical and Chemical Properties of Ellenburger Rocks, Llano County, Texas

Abstract

Physical and chemical data are recorded for 75 samples from a stratigraphic section of the Ellenburger group on Riley Mountain, Llano County, Texas. The division of samples is as follows: limestone, 29; dolomite, 42; mixed rocks, 4; a cherty limestone, a dolomitic limestone, and two calcitic dolomite samples. The limestone has a dense to sublithographic texture with pellet and breccia structures as variations. The average insoluble content of the limestone is 2.4 per cent; the minimum calcite content, about 94 per cent; and the maximum calculated dolomite content, about 3 per cent. The average grain density of 16 samples is 2.705; the average porosity of 12 samples is 0.5 per cent. Samples of the dolomite exhibit a range in texture and are classified in four groups: (1) micro-granular and very fine-grained; (2) fine-grained; (3) medium-grained; and (4) coarse-grained. Physical and chemical properties are more variable than those of the limestone samples, with ranges as follows: grain density, 2.811 to 2.848; porosity, 1.1 to 12.6 per cent; insoluble content, 0.2 to 10.2 per cent; calculated calcite content, 2.7 to 27.2 per cent; and calculated dolomite content, 70.0 to 94.7 per cent. Detrital minerals in the insoluble residues from the limestone and dolomite samples are quartz, feldspar, leucoxene, magnetite-ilmenite, tourmaline, zircon, muscovite, garnet, and apatite. Authigenic minerals include chert, quartz, feldspar, clay minerals, glauconite, and pyrite. Chert is the chief constituent of the residues, and varieties are dense or cryptocrystalline, fine to coarsely granular, and fibrous chalcedonic. The dense and coarse-grained cherts are complementary, one to the other, in their distribution. The chert, particularly of the dolomite, commonly is dolomoldic, as are also granular kaolin and glauconite. Siliceous oolites and fragments of siliceous spicules occur in a few residues. In appearance and in bulk composition the limestone samples are remarkably uniform throughout the stratigraphic section. The dolomitic limestone and the calcitic dolomite samples come from a zone which marks the transition from the dolomitic to the calcitic facies of the Staendebach member of the Tanyard formation. The coarse-grained dolomite is restricted to the lower part of the Tanyard, and the microgranular and very fine-grained dolomite is limited to the Gorman and Honeycut formations. The residues from the Tanyard formation are characterized by dense chert; whereas the residues from the Gorman and Honeycut contain abundant detrital material, principally quartz and feldspar, in addition to chert. Five detailed chemical analyses of the Ellenburger limestone and dolomite are given, and a method for determining the approximate composition of samples based on determinations of grain density and insoluble content is described. The data are presented in tables and graphically.