Saponite

noun

1. A clay mineral that is a hydrous silicate of aluminum and magnesium and occurs in soft amorphous masses in the cavities of certain rocks.
2. A hydrous silicate of magnesia and alumina. It occurs in soft, soapy, amorphous masses, filling veins in serpentine and cavities in trap-rock.
3. A hydrous silicate of magnesia and alumina. It occurs in soft, soapy, amorphous masses, filling veins in serpentine and cavities in trap rock.
4. A trioctahedral smectite mineral.