A mineral association placed in a dolomidc crystalline rock of the La Adela quarry (Province Río Negro), was studied. It was found in joints and stratification planes. The main mineral of this association is sepiolite, exceptionally well developed, and related to tale, sericite, montmorillonite and calcite. The dolomite rock presents also amphiboles and phlogopite. This mineralization is believed to be a result of hydrothermal activity, which gradually develops from an acid médium to another strongly alkaline, where the sepiolite is the last mineral formed.