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Clinoptilolite in South Texas Soils¹

ABSTRACT

Clinoptilolite, Ca_x(Na,K)_6-2x{Al₆Si₃₀O₇₂}·24H₂O, 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.

¹ Contribution from the Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX. Published with the approval of the Director of the Texas Agric. Exp. Stn. as Scientific Series Paper no. 21392.

² Research Assistant and Professor of Soil Mineralogy, Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77843. Senior author is now NRC Fellow, Mail Code SN12, NASA, Johnson Space Center, Houston, TX 77058.

Received for publication February 24, 1986.

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