Saponite is a monoclinic mineral of the montmorillonite group. Its chemical formula is (½Ca,Na)0.33(Mg,Fe+2)3(Si,Al)4O10(OH)2·4H2O.
It is soft, massive, and plastic, and exists in veins and cavities in serpentinite and basalt. The name is derived from the Greek sapo, soap. Other names include bowlingite; mountain soap; piotine; soapstone.