Specific gravity: 2.21
Colour: White, colourless
At the type locality, Rapid Creek, Dawson mining district, Yukon, Canada, rapidcreekite forms radiating acicular crystals up to 2 mm long. It occurs as a secondary phase on dilated joint-surfaces and bedding planes in a blocky, quartz-rich bed in the sideritic iron formation. Minor amounts of gypsum and aragonite are associated with rapidcreekite, and kulanite is found on the holotype specimen (CM 24.51-54).
The Diana Cave, Cerna valley, Caraş-Severin, Romania, develops along a fault line and hosts a spring of hot (51°C), sulphate-rich, sodium-calcium-chloride bearing water of near-neutral pH. Abundant steam and H2S rises from the thermal water to condensate on the walls and ceiling of the cave. The sulphuric acid produced by H2S oxidation and hydrolysis causes a strong acid-sulphate weathering of the cave bedrock generating a sulphate-dominated mineral assemblage that includes rapidcreekite closely associated with gypsum and halotrichite group minerals. Rapidcreekite forms bundles of colourless tabular orthorhombic crystals up to 1.5 mm in length (AM 98.1302-1309).
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