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The Diana Cave in SW Romania develops along a fault line and hosts a spring of hot (T avg  = 51 °C), sulfate-rich, sodium-calcium-chloride bearing water of near-neutral pH. Abundant steam and H 2 S rises from the thermal water to condensate on the walls and ceiling of the cave. The sulfuric acid produced by H 2 S oxidation/hydrolysis causes a strong acid-sulfate weathering of the cave bedrock generating a sulfate-dominated mineral assemblage that includes rapidcreekite, Ca 2 (SO 4 )(CO 3 )·4H 2 O closely associated with gypsum and halotrichite group minerals. Rapidcreekite forms bundles of colorless tabular orthorhombic crystals elongated along [001] and reaching up to 1.5 mm in length. For verifying the hydrogen bond scheme and obtaining crystal-chemical details of the carbonate group a single-crystal structure refinement of rapidcreekite was performed. Its unit-cell parameters are: a = 15.524(2), b = 19.218(3), c = 6.161(1) Å; V = 1838.1(5) Å 3 , Z = 8, space group Pcnb. Chemical composition (wt%): CaO 35.65, SO 3  24.97, CO 2  13.7, H 2 O 23.9, Na 2 O 0.291, MgO 0.173, Al 2 O 3  0.07, total 98.75%. The empirical formula, based on 7 non-water O atoms pfu, is: Ca 1.98 Na 0.029 Mg 0.013  Al 0.004 (S 0.971  O 4 )(C 0.97 O 3 )·4.13H 2 O. The δ 34 S and δ 18 O values of rapidcreekite and other cave sulfates range from 18 to 19.5‰ CDT and from -9.7 to 7.8‰ SMOW, respectively, indicating that the source of sulfur is a marine evaporite and that during hydration of the minerals it has been an abundant  18 O exchange with percolating water but almost no oxygen is derived from O 2(aq) . This is the first description of rapidcreekite from a cave environment and one of the very few natural occurrences worldwide. We also report on the mineral stability and solubility, parameters considered critical to understand the co-precipitation of carbonates and sulfates, a process that has wide applications in cement industry and scaling prevention

Rapidcreekite in the sulfuric acid weathering environment of Diana Cave, Romania