Fibers and cylinders of cryptomelane-hollandite in Permian bedded salt, Palo Duro Basin, Texas

Fibers and thin-walled, hollow cylinders of cryptomelane-hollandite have been found in both the chevron and the clear salt from various drill cores in Permian bedded salt from the Palo Duro Basin, Texas. We have examined selected core sections from the lower San Andres (units 3, 4, 5), upper San Andres, lower Clear Fork, and the Salado-Tansill Formations. We have found fibers or cylinders from only the lower San Andres Formation units 4 and 5, the upper San Andres Formation, and the Salado-Tansill salt. The fibers are inorganic, light to dark reddish brown, pleochroic, highly birefringent, filamentary single crystals, < 1 to ∼ 5 μm in diameter, with length-to-diameter ratios of at least 20:1 (some > 5000:1). Energy-dispersive analyses (sem) and Gandolfi X-ray diffraction techniques have identified the fibers and cylinders as members of the cryptomelane-hollandite series. Tunnel cations (A) of the model composition A_0-2(Mn⁴⁺,Mn³⁺)₈(O,OH)₁₆ were found to be K⁺ and Ba²⁺. Pure (within the limits of the technique) K⁺ and Ba²⁺ end members as well as intermediate compositions were observed.

The fibers can be straight and/or curved, can bifurcate, can form loops, waves or spirals, and can be isolated or in parallel groups. Detailed pétrographie analyses show no evidence for recrystallization or deformation of the enclosing salt after fiber formation. Many fibers appear to radiate from a point, line, or planar source. Although our observations do not provide a definitive explanation for fiber origin, we suggest that the fibers grew in situ by a solid-state diffusional process at low temperatures.

The cylinders are pleochroic, highly birefringent, light to dark reddish brown, hollow, thin-walled, open-ended right cylinders, having a 1- to 2-μm wall thickness and variable lengths (2 to 434 μm) and diameters (6 to 55 μm). There also appear to be single crystals of cryptomelane-hollandite, but these are found almost entirely in fluid inclusions in the chevron and clear salt. Their presence in the primary halite suggests that they were formed contemporaneously with the chevron structure and were accidentally trapped in the fluid inclusions. The cylinders found in the recrystallized salt are perhaps a residue from dissolved chevron salt.

The observation of cylinders partially or completely enclosed by salt stratigraphically above large fluid inclusions suggests that natural downward fluid-inclusion migration has occurred, in response to the geothermal gradient.