Macdonaldite is a rare barium silicate mineral with a chemical formula of BaCa4Si16O36(OH)2·10H2O.

[7] Macdonaldite was first described in 1965 and named for Gordon A. Macdonald (1911–1978) an American volcanologist at the University of Hawaii.

[8] Macdonaldite appears as veins and fracture coatings in a sanbornite and quartz bearing metamorphic rock.

Macdonaldite was first described in 1965 for an occurrence near the Big Creek-Rush Creek area in Fresno County, California.

[4][7] It has also been reported from a quarry in San Venanzo, Umbria, Italy.