Long needles and fibrous bundles of authigenic sepiolite occur in lacustrine sediments of Mound Lake, Lynn and Terry Counties, Texas. Presence of sepiolite in the fine-grained clay sediments is indicated by 12 Å X-ray diffraction, differential thermal analysis, and infrared absorption spectra.

The sepiolite is disseminated in illitic lacustrine clay immediately above a thin dolomite layer dated at 20,500 + 650 years B. P., disseminated in soft sediment dolomite some of which is apparently of Recent age, and disseminated throughout seven to eight feet of Recent playa fill.

Sepiolite probably formed from montmorillonite in the lacustrine sediments when salinity of lake brines became extreme in response to an arid climatic cycle.

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