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Sheldrickite mineral

Hydrated sodium-calcium fluorocarbonate mineral with current September 2026 IMA-CNMNC approved formula NaCa3(CO3)2F3(H2O), status A, and approval number IMA 1996-019. Grice, Gault, and Van Velthuizen (1997) described the cotype specimens (CMNMI 81530, 81537, 81538, and 81539) from thermally metamorphosed marble xenoliths at the Poudrette quarry, Mont Saint-Hilaire, Rouville County, Quebec, Canada, where it occurs on fluorite and calcite with leucophanite, pectolite, microcline, arfvedsonite, and schorlomite. Electron-microprobe WDS analysis (mean of 3 points, p. 182) combined with thermogravimetric H2O measurement and stoichiometric CO2 calculation yielded Na2O 9.16, CaO 48.84, SrO 0.36, F 16.17, CO2 25.81 (calc), H2O 5.61, less O=F 6.81, total 99.14 wt%, corresponding on the basis of 2 C atoms (10 anions) to the empirical formula Na1.01(Ca2.97Sr0.01)Sigma2.98(CO3)2[F2.90(OH)0.07]Sigma2.97·H2O. Single-crystal X-ray diffraction refined in acentric trigonal space group P32 (single-crystal cell a = 6.726(2), c = 15.044(4) A, V = 589.7(3) A3, Table 2; Debye-Scherrer powder XRD cell a = 6.718(3), c = 15.050(4) A, V = 588.3(3) A3, Z = 3, R = 4.6%, wR = 4.2% for 867 observed reflections with Fo > 5sigma(Fo) out of 965 unique reflections) established a three-layer c-repeat sequence of [CaF]-, [(CO3)2]4-, and [Na(H2O)]+ structural slabs. Thermogravimetric analysis under dry nitrogen recorded a single continuous 5.61 wt% dehydration step between 160 and 265 degrees C. Distinct from fluorite, shortite, rouvilleite, and commercial fluorspar; no criticality designation, microbial mechanism, or industrial recovery yield is asserted.

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Identifiers & Classification

System Identifier Label
IMA IMA:1996-019 Sheldrickite

Microbial & Biochemical Mechanisms (1)

Sheldrickite other Laboratory single-step thermal dehydra

Laboratory single-step thermal dehydration of natural sheldrickite

other

Heating a 2.528 mg flaky sample of natural sheldrickite from Mont Saint-Hilaire at 5 degrees C/min under dry nitrogen flowing at 200 mL/min up to 350 degrees C produced a single continuous mass loss of 0.142 mg (5.61 wt%) between 160 and 265 degrees C, matching the theoretical 5.49 wt% H2O content for 1 H2O molecule per NaCa3(CO3)2F3(H2O) formula unit.

Experimental Context & Analytical Limits:
Thermogravimetric analysis in Grice et al. (1997), pp. 182-183, used a Mettler TA3000 thermal system with a TG50 thermobalance and M3 microbalance on 2.528 mg of natural sheldrickite heated from 30 to 350 degrees C at 5 degrees C/min under 200 mL/min dry N2. Higher-temperature decarbonation above 350 degrees C and the X-ray identity of the dehydrated solid residue were not reported.
Curated Evidence Points:
Pages 182-183 report that the 2.528 mg flaky sample lost 0.142 mg (5.61 wt%) in one continuous step between 160 and 265 degrees C (versus 5.49 wt% calculated for NaCa3(CO3)2F3·H2O), confirming a single structurally distinct H2O site bonded to Na within the [Na(H2O)]+ layer.
The TGA run was terminated at 350 degrees C prior to carbonate decomposition, so CO2 (25.81 wt%) was calculated by stoichiometry rather than measured gravimetrically, and no rehydration or post-heating diffraction study was conducted.

Sources & References

  1. ima_2026_09: IMA-CNMNC, The New IMA List of Minerals, September 2026, PDF page 198, Sheldrickite row: formula NaCa3(CO3)2F3(H2O), status A, IMA No. 1996-019, country Canada, reference Canadian Mineralogist 35 (1997), 181; column definitions on PDF page 1.
    https://cnmnc.units.it/files/editor/IMA_Master_List_(2026-09).pdf (accessed 2026-10-22)
  2. grice_1997_sheldrickite: Grice JD, Gault RA, Van Velthuizen J (1997), Sheldrickite, a new sodium-calcium-fluorocarbonate mineral species from Mont Saint-Hilaire, Quebec, Canadian Mineralogist 35:181-187. Complete 7-page primary PDF visually inspected: marble-xenolith occurrence, cotypes CMNMI 81530 and 81537-81539, WDS chemical composition, and TGA dehydration pp. 181-183; powder XRD Table 1 p. 183; trigonal P32 crystal-structure refinement Tables 2-4 and Figures 3-4 pp. 184-186.
    https://www.rruff.net/doclib/cm/vol35/CM35_181.pdf (accessed 2026-10-22)

Curation History

Timestamp Curator Action Summary
2026-10-22T07:20:00Z antigravity CREATE Created sheldrickite mineral record with verified IMA:1996-019 identity, trigonal P32 crystal structure, and bounded laboratory TGA thermal dehydration mechanism from the complete 1997 primary study.

Record source: data/records/sheldrickite.yaml