A white, fibrous, flexible, and soft mineral found in a dolomitic limestone of the “La Adela” quarry, located in the Department San Antonio Oeste (Province of Rio Negro, Argentina) was studied. The mineral occurs in veinlets, placed along bedding planes and joints, showing a remarkable crystalline growth. Fibers are up to 8 cm long. In order to identify the mineral, optical microscopy, XRD, SEM, TEM, TG, DTA, IR, capacity of cationic exchange, and chemical analysis were used. It was concluded that the mineral was a sepiolite. This mineral is associated with subordinate amounts of montmorillonite, talc, phlogopite, kaolinite, and the amphibole minerals tremolite and anthophyllite. The carbonate related with the veinlets is calcite.