Palygorskite fibers growing along fault planes in the outcrops of a large fault zone in SE Spain (Carboneras Fault Zone: CFZ; Serrata de Níjar) were studied by X-ray diffraction, scanning electron microscopy-energy dispersive X-ray analysis, and transmission electron microscopy-analytical electron microscopy. The structural formulae, calculated per half unit-cell, is: Si7.95Al0.05O20(Al1.93Fe0.08Mg1.92) (OH)2(OH2)4Na0.09K0.01Ca0.034(H2O). The samples have minor tetrahedral substitutions, with Mg/Al ratios close to one, and contain very small amounts of Fe3+. The number of octahedral cations per half unit-cell is 3.93. The fault-hosted palygorskite shows macroscopic ductile features including incipient foliation. Based on field and laboratory observations, as well as on regional geological evidence indicating the existence of widespread hydrothermal processes along the Serrata de Níjar and surrounding areas, we suggest that palygorskite may have formed during ongoing deformation in the CFZ, as a precipitate from Mg-rich hydrothermal fluids.

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