Formula: BaCa4Si16O36(OH)2.10H2O
Phyllosilicate (sheet silicate), barium-bearing mineral
Crystal System: Orthorhombic
Specific gravity: 2.27 measured, 2.27 calculated
Hardness: 3½ to 4
Streak: White
Colour: White, colourless
Solubility: Insoluble in cold acids, but decomposed by boiling hydrochloric acid AM 50.314-340
Common impurities: Ti,Al,Fe,Mn,Mg,Sr

Metamorphic environments
Hydrothermal environments


There are three co-type localities, all at Big Creek, Rush Creek deposit, Big Creek-Rush Creek Mining Fresno county, California, USA. Macdonaldite is typically a vein-forming mineral, and it is found within or closely associated with sanbornite-quartz rock both here at the type locality and at the sanbornite type locality north of Incline, Mariposa county, California. Macdonaldite forms small radiating crystal aggregates up to 12 mm in diameter and individual fibres as much as 6 mm long, but more commonly only about 1 mm long. Macdonaldite locally crystallises along cleavage surfaces of sanbornite and coats joints and fractures as well as forming discrete veinlets.
Other barium-bearing silicate minerals found here within or closely associated with sanbornite-quartz rocks include krauskopfite, walstromite, fresnoite, verplanckite, muirite and traskite (AM 50.314-340).

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