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Margaritasite mineral

Hydrated cesium-uranyl divanadate mineral of the carnotite group with current September 2026 IMA-CNMNC approved formula Cs2(UO2)2(VO4)2·H2O (structural formula Cs2(UO2)2V2O8·H2O), status A, and approval number IMA 1980-093. Wenrich, Modreski, Zielinski, and Seeley (1982) described the type specimen (deposited at the U.S. National Museum of Natural History, Smithsonian Institution, Washington, D.C.) from the Margaritas uranium-molybdenum deposit (site 19 and drill hole #33), Sierra Pena Blanca Uranium District, Chihuahua, Mexico, where it occurs as 1-3 um yellow tabular plates filling matrix pores and lining leached phenocryst molds in altered Oligocene rhyodacitic tuff breccia of the lower Escuadra Formation alongside quartz, feldspar, kaolinite, pyrite, and barite. ARL-SEMQ electron-microprobe analysis of 14 grains (Table 4, p. 1282) yielded an anhydrous mean of UO3 55.4, Cs2O 17.6, V2O5 17.0, K2O 1.63, Ca 0.33, Ba 0.19, Si 0.14, Sr 0.07, Al 0.02, sum 91.63 wt% (recalculated with 2.5 wt% H2O to 100%: UO3 58.9, Cs2O 18.7, V2O5 18.1, K2O 1.73 wt%), while AAS, ICAP-AES, and CHN wet-chemical analysis of three ultrasonic-concentrated splits (containing ~11 wt% admixed quartz, Table 4) yielded UO3 49.6, Cs2O 17.0, V2O5 15.9, H2O 2.5, K2O 1.20 wt% (recalculated quartz-free to 100%: UO3 57.5, Cs2O 19.7, V2O5 18.4, H2O 2.9, K2O 1.39 wt%), corresponding to the empirical formula (Cs1.38(H3O)0.35K0.29)Sigma2.02(UO2)1.99(V2O8)1.00·1.07H2O. X-ray powder diffraction (Tables 2-3, pp. 1279-1280) refined in monoclinic space group P21/a (Z = 2) gave a = 10.514(3), b = 8.425(3), c = 7.252(5) A (7.25 A in abstract), beta = 106.01 degrees, V = 617.48(44) A3, and Dcalc = 5.41 g/cm3 (5.40 g/cm3 in Table 3; biaxial negative, nalpha ~ 1.797 calc, nbeta = 1.925-1.930, ngamma = 1.945-1.950, 2V = 42(5) degrees). Hydrothermal CsCl ion-exchange experiments at 200 degrees C converted natural carnotite into margaritasite. Distinct from carnotite (K2(UO2)2(VO4)2·3H2O), tyuyamunite, sengierite, and elemental cesium, vanadium, or uranium; no criticality designation, microbial mechanism, or industrial recovery yield is asserted.

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Identifiers & Classification

System Identifier Label
IMA IMA:1980-093 Margaritasite

Microbial & Biochemical Mechanisms (1)

Margaritasite other Laboratory hydrothermal Cs+ ion-exchan

Laboratory hydrothermal Cs+ ion-exchange conversion of natural carnotite to margaritasite

other

Hydrothermal reaction of 10 mg natural Colorado Plateau carnotite (0.12 wt% Cs2O, K2(UO2)2V2O8·nH2O, c = 6.59 A) with 0.05-0.10 g of 0.1 M CsCl aqueous solution (pH 5.6) in sealed Pt capsules at 200 degrees C for 14, 61, and 209 days converted carnotite via interlayer K+ -> Cs+ ion exchange into discrete crystalline margaritasite characterized by a sharp (001) diffraction peak at 12.7 degrees 2theta (d = 7.0 A, c = 7.30-7.31 A) and bulk Cs2O uptake reaching 10.67-12.83 wt% (with individual Cs-rich crystal domains exceeding Cs/(K+Cs) = 0.70 by EMPA); by contrast, parallel runs at 80 degrees C for 14 and 61 days produced only 0.75-0.85 wt% Cs2O uptake (mole fraction Cs/(K+Cs) < 0.05) with no margaritasite phase transition, and heating natural margaritasite in air from 100 to 500 degrees C for 68 h contracted c from 7.25 to 7.17 A.

Experimental Context & Analytical Limits:
Hydrothermal capsule ion-exchange and heating experiments reported by Wenrich et al. (1982), pp. 1281 and 1283-1287, Tables 5-7, and Figures 5-6, constraining the >100-200 degrees C hydrothermal origin of natural margaritasite at Sierra Pena Blanca.
Curated Evidence Points:
Tables 5-7 and Figures 5-6 (pp. 1283-1286) demonstrate that 200 degrees C hydrothermal reaction of carnotite with 0.1 M CsCl for 14, 61, and 209 days produces the 12.7 degrees 2theta (001) margaritasite reflection (c = 7.30-7.31 A) and bulk Cs2O contents of 10.67-12.83 wt% (with Cs/(K+Cs) > 0.7 in margaritasite domains), whereas 80 degrees C runs yield only 0.75-0.85 wt% Cs2O without phase change.
Even after 209 days at 200 degrees C, XRD patterns and EMPA traverses (Table 7, Figure 5) show an inhomogeneous two-phase mixture of Cs-rich margaritasite domains and residual carnotite domains because a miscibility gap separates carnotite and margaritasite across 0.06 <= Cs/(K+Cs) <= 0.70.

Sources & References

  1. ima_2026_09: IMA-CNMNC, The New IMA List of Minerals, September 2026, PDF page 136, Margaritasite row: formula Cs2(UO2)2(VO4)2·H2O, status A, IMA No. 1980-093, country Mexico, reference American Mineralogist 67 (1982), 1273; column definitions on PDF page 1.
    https://cnmnc.units.it/files/editor/IMA_Master_List_(2026-09).pdf (accessed 2026-10-23)
  2. wenrich_1982_margaritasite: Wenrich KJ, Modreski PJ, Zielinski RA, Seeley JL (1982), Margaritasite: a new mineral of hydrothermal origin from the Pena Blanca Uranium District, Mexico, American Mineralogist 67:1273-1289. Complete 17-page primary PDF visually inspected: Margaritas deposit occurrence and paragenesis pp. 1273-1278; monoclinic P21/a powder XRD and heating contraction of c Tables 2-3 pp. 1278-1281; EMPA and wet-chemical analyses Table 4 p. 1282; hydrothermal CsCl ion-exchange synthesis from carnotite at 80 and 200 C Tables 5-7 and Figures 5-6 pp. 1283-1287.
    https://www.rruff.net/doclib/am/vol67/AM67_1273.pdf (accessed 2026-10-23)

Curation History

Timestamp Curator Action Summary
2026-10-23T07:15:00Z antigravity CREATE Created margaritasite mineral record with verified IMA:1980-093 identity, Margaritas deposit EMPA and wet-chemical compositions, monoclinic P21/a crystallography, and bounded hydrothermal Cs+ ion-exchange mechanism from the complete 1982 primary study.

Record source: data/records/margaritasite.yaml