The invention provides a composition of particulate materials. The composition comprises lanthanum chloride in particulate form. The composition also comprises up to about 4% by weight of amorphous silica in particulate form, based on the combined weight of the lanthanum chloride and the amorphous silica. The addition of amorphous silica to desiccated lanthanum chloride forms a fine coating or barrier on the outer surfaces of the individual lanthanum crystals, providing a composition that is significantly more stable and able to resist coalescence of particles than pure desiccated lanthanum chloride.