Asbestos from the Diablo Prospect, Allamore Talc District (Texas) has cell dimensions a 9.944(5), B 17.951(6), C 5.271(4) A, B 104.41(5)Deg (powder-diffraction data); the data are consistent with space groups C2, Cm, or C2/m. Chemical analyses indicated a potassian winchite-asbestos, (k0.43Na0.02) (Na1.12Ca0.68Mg0.20) (Mg4.82A10.11Fe3+0.07) (Si7.99A10.01) O21.55(OH1.88F0.51). Low indices of refraction, na'=1.576 to 1.596, NV'=1.590 to 1.600, are probably related to the presence of fluorine and the low Ca and Fe contents. Edax indicates that their variability is related to Na, K, Ca, and Mg contents. In composition, this asbestos probably ranges from a potassian richterite to a potassian winchite. The asbestos habit is here well developed: fibers are composed of fibrils 200 to 400 A wide, oriented with C axes parallel, but with no preferred orientation of A and B. This fibril-bundle structure explains the optical properties of parallel extinction and anomalous interference figures. This asbestos is considered a product of low-temperature metasomatism of a siliceous dolomite. The chemical and physical characteristics described here are consistent with this interpretation.