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

Bogdan P. Onac, Herta S. Effenberger, Jonathan G. Wynn and Ioan Povara
Rapidcreekite in the sulfuric acid weathering environment of Diana Cave, Romania
American Mineralogist (July 2013) 98 (7): 1302-1309


The Diana Cave in SW Romania develops along a fault line and hosts a spring of hot (T (sub avg) = 51 degrees C), sulfate-rich, sodium-calcium-chloride bearing water of near-neutral pH. Abundant steam and H (sub 2) S rises from the thermal water to condensate on the walls and ceiling of the cave. The sulfuric acid produced by H (sub 2) S oxidation/hydrolysis causes a strong acid-sulfate weathering of the cave bedrock generating a sulfate-dominated mineral assemblage that includes rapidcreekite, Ca (sub 2) (SO (sub 4) )(CO (sub 3) ).4H (sub 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) Aa; V = 1838.1(5) Aa (super 3) , Z = 8, space group PCNB: Chemical composition (wt%): CaO 35.65, SO (sub 3) 24.97, CO (sub 2) 13.7, H (sub 2) O 23.9, Na (sub 2) O 0.291, MgO 0.173, Al (sub 2) O (sub 3) 0.07, total 98.75%. The empirical formula, based on 7 non-water O atoms pfu, is: Ca (sub 1.98) Na (sub 0.029) Mg (sub 0.013) Al (sub 0.004) (S (sub 0.971) O (sub 4) )(C (sub 0.97) O (sub 3) ).4.13H (sub 2) O. The delta (super 34) S and delta (super 18) O values of rapidcreekite and other cave sulfates range from 18 to 19.5 per mil CDT and from -9.7 to 7.8 per mil SMOW, respectively, indicating that the source of sulfur is a marine evaporite and that during hydration of the minerals it has been an abundant (super 18) O exchange with percolating water but almost no oxygen is derived from O (sub 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.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 98
Serial Issue: 7
Title: Rapidcreekite in the sulfuric acid weathering environment of Diana Cave, Romania
Affiliation: University of South Florida, Department of Geology, Tampa, FL, United States
Pages: 1302-1309
Published: 201307
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 62
Accession Number: 2013-061403
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
N43°40'00" - N48°10'00", E20°15'00" - E29°45'00"
Secondary Affiliation: Babes-Bolyai University, ROM, RomaniaUniversitaet Wien, AUT, Austria
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201337
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