Clinoptilolite in South Texas Soils

Abstract

Abstract Clinoptilolite, Ca x (Na,K) 6‐2 x {Al 6 Si 30 O 72 }·24H 2 O, frequently has been recognized in Cenozoic volcanogenic sedimentary rocks; however, its occurrence in soils has received little attention. In the course of this study, clinoptilolite was identified in calcareous soils derived from volcanic‐rich sediments of the Catahoula Formation in south Texas. The mineralogical properties of clinoptilolite were characterized by x‐ray diffraction (XRD), scanning electron microscopy (SEM), electron microprobe analysis, and analysis of exchangeable cations. About 2 to 5% clinoptilolite by weight was found in the A and B horizons and concentrations progressively increased with depth to as high as 20% in the C horizons. No differences in the zeolite structural framework or Si/Al atomic ratio were detected throughout the soil profiles examined. Unlike Na‐ and K‐saturated clinoptilolites in sedimentary deposits, clinoptilolite in these calcareous soils was predominantly Ca‐saturated. Clinoptilolite was concentrated in the silt fractions as clusters of closely spaced laths that were 1 to 2 µm in thickness and 5 to 10 µm in length. Aggregates of the crystal plates and laths were as large as 50 to 60 µm in diam. Clinoptilolite in these soils appeared to have been inherited from zeolite‐rich, tuffaceous parent materials. The smooth, angular, euhedral nature of the clinoptilolite crystals indicated their relative stability. Exceptional, slightly pitted and fractured crystals were observed in the A and B horizons indicating minimal weathering.