a complex hydrous silicate of aluminum and magnesium, a light-colored, soft clay mineral, often found in veins and cavities in serpentine and basaltic rocks
See saponite in American Heritage Dictionary 4
(săpˈə-nītˌ)
noun
A clay mineral that is a hydrous silicate of aluminum and magnesium and occurs in soft amorphous masses in the cavities of certain rocks.