Persistence of a Zeolite in Tuffaceous Soils of the Texas Trans-Pecos

Abstract

Clinoptilolite, a highly siliceous zeolite, was observed in tuffaceous sediments of the Texas Trans-Pecos volcanic field. A toposequence of three soils (a Ustollic Calciorthid, a Lithic Torriorthent, and a Ustollic Haplargid) was selected to study the pedogenic fate of clinoptilolite formed in the tuff. X-ray diffraction and electron microscopy were used to determine the mineralogy of soil particlesize separates. Clinoptilolite was observed in the silt (0.002–0.05 mm) and coarse clay (0.0002–0.002 mm) of all horizons, including the tuff. Well-crystallized clinoptilolite was not detected in the fine clay (<0.0002 mm) of the near-surface horizons of the pedons on nonerosional positions, but was present in the fine clay of deeper horizons in these soils. Scanning electron microscopy revealed that most of the clinoptilolite occurred in the interiors of volcanic glass-shard pseudomorphs. The shard pseudomorphs were postulated to act as a shielding mechanism protecting clinoptilolite from intensive weathering. The fact that well-crystallized clinoptilolite was not found in the fine clay of the surficial horizons suggests that, when the shards are fractured or weathered, clinoptilolite weathers rather quickly, at least in the finer fractions. Clinoptilolite cannot, therefore, be considered a stable mineral in this environment, although, because of its mode of formation, it is remarkably persistent.