An improved finely divided rubber-reinforcing clay mineral composition can be produced by employing a hydrous kaolin clay surface treated with a functional silane, a methylene donor compound such as hexamethylenetetramine, and a multifunctional methylene acceptor such as resorcinol. A novel process of preparation is provided. The improved reinforcing, treated clay filler can be combined with natural or synthetic elastomer compositions which can be cured to elastomer products which have improved physical properties, such as modulus, tensile, tear, wear resistance and/or resistance to heat build-up.