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Boltwoodite mineral

Boltwoodite is a potassium-bearing uranyl silicate hydrate mineral, distinct from sodium-dominant natroboltwoodite and elemental uranium. The September 2026 IMA list gives (K0.63Na0.37)(UO2)(SiO3OH)(H2O)1.5, status G and original-description year 1956; the year is not an approval accession. This listed composition is not imposed on every specimen. Burns's 1998 natural crystal from near Rössing, Namibia, refined in monoclinic P21/m with structural formula (K0.56Na0.42)[(UO2)(SiO3OH)](H2O)1.5. The formula reflects partially occupied interlayer sites; water in the accompanying microprobe table was calculated from stoichiometry. Vochten and colleagues instead studied synthetic potassium-rich material and reported an ideal one-water formula and empirical 1.30 waters per formula unit. Its preparation, transformation and aqueous-release experiments below retain that source-specific material identity. No criticality, microbial mechanism or industrial recovery claim is inferred.

Microbial & Biochemical Mechanisms (2)

Abiotic hydrothermal formation and later transformation to soddyite

other

Synthetic boltwoodite formed from uranyl nitrate, potassium chloride and quartz in alkaline hydrothermal solution. Longer reaction times favored soddyite. The experiment supports laboratory phase formation and transformation, without a measured isolated uranium recovery yield or a demonstrated molecular pathway.

Experimental Context & Analytical Limits:
Vochten et al. used 0.35 g UO2(NO3)2·6H2O and 0.5 g KCl in 25 mL water adjusted to pH 11.5 with KOH, with 3 g synthetic hydrothermal quartz chunks approximately 1 cm across. A Teflon vessel in a Parr reaction bomb was held at 185 degrees C for seven days, with a calculated pressure of approximately 1.3 MPa. After cooling, quartz fragments were removed; colloid and yellow prismatic material were removed by three washes with 5% acetic acid, followed by distilled water washing. Bromoform separation removed smaller quartz particles. The remaining air-dried needles were identified by chemical analysis and powder X-ray diffraction. A fused-silica-crucible experiment reportedly gave the same result; a numerical replicate count was not supplied.
Curated Evidence Points:
Page 736 reports maximum boltwoodite yield after seven days, followed by decreasing yield as soddyite appeared; runs of 40 days or more contained only soddyite crystals. Separate experiments suspending natural and synthetic boltwoodite in distilled water under the same hydrothermal conditions confirmed slow transformation. The paper does not provide a numerical yield curve or phase fractions, or specify the separate water-arm loading, volume, pH or exact conversion time. The synthesis pH and reagent quantities are not transferred to those separate water experiments.
Table 1 and page 737 concern the synthetic product and give K0.92(UO2)1.03(SiO3OH)0.96·1.30H2O and ideal K(UO2)(SiO3OH)·H2O. These reported formulae are retained separately from the current IMA entry and the natural 1998 crystal. The Ca, Mg, Pb and Na salt treatments are separate repeated-cycle arms; their products and qualitative potassium endpoints do not quantify this synthesis or water-only transformation. No microbes or industrial plant operation were tested.

Abiotic pH-dependent aqueous uranium release from synthetic boltwoodite

other

Synthetic boltwoodite released more uranium into acidic water than into the less acidic and alkaline solutions tested. The measured filtered-solution total uranium concentrations are not free uranyl activities, a dissolution rate, a recovery percentage or a determined thermodynamic solubility product.

Experimental Context & Analytical Limits:
Synthetic boltwoodite aliquots in carbon-dioxide-free distilled water were adjusted with KOH or HClO4 over nominal pH 3.8-11.8 and shaken at 25 degrees C under nitrogen. The authors report constant total solution uranium after 30 days, followed by filtration through a 1.4 micrometre Millipore filter. Uranium was measured with Arsenazo III spectrophotometry at 662.5 nm. Solid loading, solution volume, numerical replicate count and concentration uncertainties are not supplied for this arm.
Curated Evidence Points:
Table 3 on page 739 reports U concentration in g of U per litre: 4.8 × 10^-3 at pH 3.81, 1.7 × 10^-3 at pH 4.31, 1.3 × 10^-4 at pH 4.81, 1.3 × 10^-6 at pH 5.75, 1.3 × 10^-6 at pH 7.38, 1.2 × 10^-6 at pH 7.60, and 1.2 × 10^-6 at pH 11.73. Page 740 reports only boltwoodite lines in powder diffraction of the air-dried post-experiment solids. These measurements belong to this synthetic potassium-rich material, not sodium boltwoodite or cuprosklodowskite.
Page 740 reports visible silica gel below pH 7.5 after equilibration and possible earlier colloidal silica formation. Unresolved silicate speciation prevented an accurate solubility-product calculation; the authors therefore report total uranium only. Constant measured uranium and retained diffraction lines do not by themselves prove reversible equilibrium or exclude colloids passing the stated filter.

Sources & References

  1. ima_2026_09: IMA-CNMNC. The New IMA List of Minerals, September 2026. Boltwoodite row on PDF page 28, status/year definitions on page 1. Official current list and homepage freshly acquired; mineral row visually checked. The row cites Science 124 (1956), 931 and American Mineralogist 110 (2025), 1318. The latter article's full text was unavailable in this investigation; no experimental claim from it is accepted here.
    https://cnmnc.units.it/files/editor/IMA_Master_List_(2026-09).pdf (accessed 2026-10-16)
  2. burns1998: Burns PC (1998). The structure of boltwoodite and implications of solid solution toward sodium boltwoodite. The Canadian Mineralogist 36:1069-1075. Complete original seven-page article acquired and visually inspected. Natural specimen and analytical methods, pages 1070-1071; Tables 1-5 and Figures 1-3; structural formula, page 1074. Cited CISTI observed/calculated structure-factor deposit was not retrieved. No DOI was verified in the bounded search.
    https://www.rruff.net/doclib/cm/vol36/CM36_1069.pdf (accessed 2026-10-16)
  3. vochten1997: Vochten R, Blaton N, Peeters O, Van Springel K, Van Haverbeke L (1997). A new method of synthesis of boltwoodite and of formation of sodium boltwoodite, uranophane, sklodowskite and kasolite from boltwoodite. The Canadian Mineralogist 35:735-741. Complete original seven-page article, Tables 1-4 and Figures 1-3 re-inspected from the retained public RRUFF copy. Synthesis page 736; empirical/ideal formulae page 737; aqueous-release methods and Table 3 pages 739-740. No DOI was verified in the bounded identifier search.
    https://www.rruff.net/doclib/cm/vol35/CM35_735.pdf (accessed 2026-10-16)

Curation History

Timestamp Curator Action Summary
2026-10-16T02:41:23.120299Z codex (GPT-6; codex) CREATE Added current mineral identity with specimen-specific natural and synthetic compositions, laboratory hydrothermal transformation and measured total aqueous uranium with silica-speciation limits.

Record source: data/records/boltwoodite.yaml