Boltwoodite
A valid IMA mineral species - grandfathered
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About Boltwoodite
Bertram Borden Boltwood
Formula:
(K,Na)(UO2)(SiO3OH) · 1.5H2O
Colour:
Pale yellow, orange yellow
Lustre:
Sub-Vitreous, Waxy, Silky
Hardness:
3½ - 4
Specific Gravity:
4.7
Crystal System:
Monoclinic
Member of:
Name:
For Bertram Borden Boltwood (1870-1927), radiochemist at Yale University, New Haven, Connecticut, USA. Named by Clifford Frondel and Jun Ito.
Isostructural with:
Unique Identifiers
Mindat ID:
716
Long-form identifier:
mindat:1:1:716:0
IMA Classification of Boltwoodite
Approved, 'Grandfathered' (first described prior to 1959)
IMA Formula:
K0.63Na0.37[(U6+O2)(SiO3OH)](H2O)1.5
First published:
1956
Classification of Boltwoodite
9.AK.15
9 : SILICATES (Germanates)
A : Nesosilicates
K : Uranyl neso- and polysilicates
9 : SILICATES (Germanates)
A : Nesosilicates
K : Uranyl neso- and polysilicates
53.3.1.5
53 : NESOSILICATES Insular SiO4 Groups and Other Anions or Complex Cations
3 : Insular SiO4 Groups and Other Anions of Complex Cations with (UO2)
53 : NESOSILICATES Insular SiO4 Groups and Other Anions or Complex Cations
3 : Insular SiO4 Groups and Other Anions of Complex Cations with (UO2)
14.16.6
14 : Silicates not Containing Aluminum
16 : Silicates of U
14 : Silicates not Containing Aluminum
16 : Silicates of U
Mineral Symbols
As of 2021 there are now IMA–CNMNC approved mineral symbols (abbreviations) for each mineral species, useful for tables and diagrams.
| Symbol | Source | Reference for Standard |
|---|---|---|
| Bwd | IMA–CNMNC | Warr, L.N. (2021). IMA–CNMNC approved mineral symbols. Mineralogical Magazine, 85(3), 291-320. doi:10.1180/mgm.2021.43 |
Physical Properties of Boltwoodite
Sub-Vitreous, Waxy, Silky
Transparency:
Translucent
Comment:
Aggregates vitreous to silky, dull to earthy in pseudomorphs
Colour:
Pale yellow, orange yellow
Streak:
White
Hardness:
3½ - 4 on Mohs scale
Tenacity:
Brittle
Cleavage:
Perfect
Perfect on {010}, imperfect on {001}.
Perfect on {010}, imperfect on {001}.
Fracture:
Irregular/Uneven
Density:
4.7 g/cm3 (Measured) 4.46 g/cm3 (Calculated)
Optical Data of Boltwoodite
Type:
Biaxial (-)
RI values:
nα = 1.668 - 1.670 nβ = 1.695 - 1.696 nγ = 1.698 - 1.703
Birefringence:
0.007-0.008
Max. Birefringence:
δ = 0.030 - 0.033
Based on recorded range of RI values above.
Based on recorded range of RI values above.
Interference Colours:
The colours simulate birefringence patterns seen in thin section under crossed polars. They do not take into account mineral colouration or opacity.
Michel-Levy Bar The default colours simulate the birefringence range for a 30 µm thin-section thickness. Adjust the slider to simulate a different thickness.
Grain Simulation You can rotate the grain simulation to show how this range might look as you rotated a sample under crossed polars. Each grain retains its interference colour (retardation) while its brightness falls to black at extinction and reaches a maximum between extinction positions.
The colours simulate birefringence patterns seen in thin section under crossed polars. They do not take into account mineral colouration or opacity.
Michel-Levy Bar The default colours simulate the birefringence range for a 30 µm thin-section thickness. Adjust the slider to simulate a different thickness.
Grain Simulation You can rotate the grain simulation to show how this range might look as you rotated a sample under crossed polars. Each grain retains its interference colour (retardation) while its brightness falls to black at extinction and reaches a maximum between extinction positions.
Surface Relief:
Very High (positive)
Relative to Canada balsam mounting medium (n ≈ 1.537).
Relative to Canada balsam mounting medium (n ≈ 1.537).
This shows the grain boundary and Becke line effect under plane-polarised
light, based on the contrast between this mineral's average refractive
index and the mounting medium. It does not take into account mineral
colouration.
In focus
Interference Figure:
This shows the idealized biaxial acute bisectrix (Bxa) interference figure
- the conoscopic view for a grain cut perpendicular to the acute bisectrix, using
this mineral's 2V. The two small white dots mark the melatopes - the points
where the two optic axes emerge - and are shown only when they fall within the
field of view. The coloured bands are isochromatics, and the dark bands are
isogyres.
Rotate the stage: at 0°/90° the isogyres form a cross through the melatopes; at 45° they pull apart into curved hyperbolas. That splitting on rotation - absent in a uniaxial figure - is the standard diagnostic test for telling biaxial minerals from uniaxial ones. If 2V is large, the melatopes may fall outside the field of view, as they often do at the microscope too.
No measured or calculated 2V is on file for this mineral, so the value used here (46°) is estimated from its recorded refractive indices and optic sign, not from a direct 2V measurement.
Rotate the stage: at 0°/90° the isogyres form a cross through the melatopes; at 45° they pull apart into curved hyperbolas. That splitting on rotation - absent in a uniaxial figure - is the standard diagnostic test for telling biaxial minerals from uniaxial ones. If 2V is large, the melatopes may fall outside the field of view, as they often do at the microscope too.
No measured or calculated 2V is on file for this mineral, so the value used here (46°) is estimated from its recorded refractive indices and optic sign, not from a direct 2V measurement.
Dispersion:
r > v or r < v
Pleochroism:
Weak
Comments:
X = colorless; Y = Z = yellow.
Chemistry of Boltwoodite
Mindat Formula:
(K,Na)(UO2)(SiO3OH) · 1.5H2O
Element Weights:
Common Impurities:
Fe,Ca,Na
Crystallography of Boltwoodite
Crystal System:
Monoclinic
Class (H-M):
2 - Sphenoidal
Space Group:
P21
Cell Parameters:
a = 7.073(2) Å, b = 7.064(1) Å, c = 6.638(1) Å
β = 105.45°
β = 105.45°
Ratio:
a:b:c = 1.001 : 1 : 0.94
Unit Cell V:
319.67 ų (Calculated from Unit Cell)
Z:
2
Morphology:
Radiating acicular to fibrous crystals, also earthy pseudomorphs of uranium minerals.
Crystal Structure
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Data courtesy of the American Mineralogist Crystal Structure Database. Click on an AMCSD ID to view structure
| ID | Species | Reference | Link | Year | Locality | Pressure (GPa) | Temp (K) |
|---|---|---|---|---|---|---|---|
| 0000838 | Boltwoodite | Stohl F V, Smith D K (1981) The crystal chemistry of the uranyl silicate minerals American Mineralogist 66 610-625 |  | 1981 | New Method mine, Amboy, California, USA | 0 | 293 |
| 0005562 | Boltwoodite | Burns P C (1998) The structure of boltwoodite and implications of solid solution toward sodium boltwoodite The Canadian Mineralogist 36 1069-1075 | | 1998 | 0 | 293 |
CIF Raw Data - click here to close
X-Ray Powder Diffraction
Powder Diffraction Data:
| d-spacing | Intensity |
|---|---|
| 6.81 Å | (10) |
| 6.40 Å | (50) |
| 3.54 Å | (70) |
| 3.40 Å | (90) |
| 2.95 Å | (80) |
| 2.91 Å | (70) |
| 1.900 Å | (60) |
| 1.764 Å | (60) |
Comments:
35-490
Geological Environment
Paragenetic Mode(s):
| Paragenetic Mode | Earliest Age (Ga) |
|---|---|
| Near-surface Processes | |
| 23 : Subaerial aqueous alteration by non-redox-sensitive fluids (see also #47) | |
| Stage 7: Great Oxidation Event | <2.4 |
| 47a : [Near-surface hydration of prior minerals] | |
| 47f : [Uranyl (U⁶⁺) minerals] |
Geological Setting:
Alteration zone around uranium deposits.
Type Occurrence of Boltwoodite
General Appearance of Type Material:
Wart-like aggregates of fibers coating fractures in sandstone.
Place of Conservation of Type Material:
National Museum of Natural History, Washington, D.C., USA, 112710.
Geological Setting of Type Material:
An oxidation product of primary black U ores.
Associated Minerals at Type Locality:
Other Language Names for Boltwoodite
Relationship of Boltwoodite to other Species
Member of:
Other Members of Uranophane Group:
| Natroboltwoodite | Na(UO2)(SiO3OH) · H2O | Orth. 222 : P212121 |
| Parauranophane | Ca(UO2)2(SiO3OH)2 · 5H2O | Mon. 2/m : P21/b |
| Uranophane | Ca(UO2)2(SiO3OH)2 · 5H2O | Mon. 2 : P21 |
Common Associates
Associations Based on Photo Data:
| 109 photos of Boltwoodite associated with Calcite | CaCO3 |
| 29 photos of Boltwoodite associated with Fluorite | CaF2 |
| 17 photos of Boltwoodite associated with Uraninite | UO2 |
| 11 photos of Boltwoodite associated with Gypsum | CaSO4 · 2H2O |
| 9 photos of Boltwoodite associated with Quartz | SiO2 |
| 8 photos of Boltwoodite associated with Zippeite | K3(UO2)4(SO4)2O3(OH) · 3H2O |
| 6 photos of Boltwoodite associated with Coffinite | U(SiO4) · nH2O |
| 4 photos of Boltwoodite associated with Haynesite | (UO2)3(Se4+O3)2(OH)2 · 5H2O |
| 4 photos of Boltwoodite associated with Allanite-(Y) | (CaY)(AlAlFe2+)O[Si2O7][SiO4](OH) |
| 3 photos of Boltwoodite associated with Brochantite | Cu4(SO4)(OH)6 |
Related Minerals - Strunz-mindat Grouping
| 9.AK. | 'Orlite' | Pb3(UO2)3(Si2O7)2 · 6H2O |
| 9.AK.05 | Soddyite | (UO2)2SiO4 · 2H2O |
| 9.AK.10 | Sklodowskite | Mg(UO2)2(SiO3OH)2 · 6H2O |
| 9.AK.10 | Cuprosklodowskite | Cu(UO2)2(SiO3OH)2 · 6H2O |
| 9.AK.10 | Oursinite | Co(UO2)2(SiO3OH)2 · 6H2O |
| 9.AK.15 | Parauranophane | Ca(UO2)2(SiO3OH)2 · 5H2O |
| 9.AK.15 | Uranophane | Ca(UO2)2(SiO3OH)2 · 5H2O |
| 9.AK.15 | Natroboltwoodite | Na(UO2)(SiO3OH) · H2O |
| 9.AK.15 | Kasolite | Pb(UO2)(SiO4) · H2O |
| 9.AK.20 | Swamboite-(Nd) | Nd0.333[(UO2)(SiO3OH)](H2O)~2.5 |
| 9.AK.25 | Haiweeite | Ca(UO2)2[Si5O12(OH)2] · 6H2O |
| 9.AK.25 | Metahaiweeite | Ca(UO2)2Si6O15 · nH2O |
| 9.AK.30 | Weeksite | K2(UO2)2(Si5O13) · 4H2O |
| 9.AK.30 | Coutinhoite | ThxBa(1-2x)(UO2)2Si5O13 · (H2O)1+y (0 < x < 0.5 and 0 < y < (2+x)) |
| 9.AK.30 | Barronite | (◻0.5Ba0.5)(UO2)2Si5O12(OH) · 2H2O |
| 9.AK.35 | Magnioursilite | Mg4(UO2)4(Si2O5)5(OH)6 · 20H2O |
| 9.AK.35 | Calcioursilite | Ca4(UO2)4(Si2O5)5(OH)6 · 15H2O |
| 9.AK.40 | Uranosilite | UO3 · 7SiO2 |
Radioactivity
Radioactivity:
| Element | % Content | Activity (Bq/kg) | Radiation Type |
|---|---|---|---|
| Uranium (U) | 55.4536% | 13,863,400 | α, β, γ |
| Thorium (Th) | 0.0000% | 0 | α, β, γ |
| Potassium (K) | 9.1087% | 2,824 | β, γ |
For comparison:
- Banana: ~15 Bq per fruit
- Granite: 1,000–3,000 Bq/kg
- EU exemption limit: 10,000 Bq/kg
Note: Risk is shown relative to daily recommended maximum exposure to non-background radiation of 1000 µSv/year. Note that natural background radiation averages around 2400 µSv/year so in reality these risks are probably extremely overstated! With infrequent handling and safe storage natural radioactive minerals do not usually pose much risk.
Interactive Simulator:
Note: The mass selector refers to the mass of radioactive mineral present, not the full specimen, also be aware that the matrix may also be radioactive, possibly more radioactive than this mineral!
Activity: –
| Distance | Dose rate | Risk |
|---|---|---|
| 1 cm | ||
| 10 cm | ||
| 1 m |
The external dose rate (D) from a radioactive mineral is estimated by summing the gamma radiation contributions from its Uranium, Thorium, and Potassium content, disregarding daughter-product which may have a significant effect in some cases (eg 'pitchblende'). This involves multiplying the activity (A, in Bq) of each element by its specific gamma ray constant (Γ), which accounts for its unique gamma emissions. The total unshielded dose at 1 cm is then scaled by the square of the distance (r, in cm) and multiplied by a shielding factor (μshield). This calculation provides a 'worst-case' or 'maximum risk' estimate because it assumes the sample is a point source and entirely neglects any self-shielding where radiation is absorbed within the mineral itself, meaning actual doses will typically be lower. The resulting dose rate (D) is expressed in microsieverts per hour (μSv/h).
D = ((AU × ΓU) + (ATh × ΓTh) + (AK × ΓK)) / r2 × μshield
Other Information
Health Risks:
Contains uranium - always wash hands after handling. Avoid inhaling dust when handling or breaking. Never lick or ingest. Avoid prolonged exposure in proximity of the body. Store away from inhabited areas.
Internet Links for Boltwoodite
mindat.org URL:
https://www.mindat.org/min-716.html
Please feel free to link to this page.
Please feel free to link to this page.
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External Links:
Mineral Dealers:
References for Boltwoodite
Reference List:
FRONDEL, C., ITO, J. (1956) Boltwoodite, a New Uranium Silicate. Science, 124 (3228). 931 doi:10.1126/science.124.3228.931
Honea, Russell M. (1961) New data on boltwoodite, an alkali uranyl silicate. American Mineralogist, 46 (1-2) 12-25
KATO, TOSHIO, MIURA, YASUNORI (1974) Cell Dimensions of Boltwoodite. Mineralogical Journal, 7 (4). 400-404 doi:10.2465/minerj1953.7.400
Stohl, Frances V., Smith, Deane K. (1981) The crystal chemistry of the uranyl silicate minerals. American Mineralogist, 66 (5-6) 610-624
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 (3) 735-741
Burns, P. C. (1998) The structure of boltwoodite and implications of solid-solution towards sodium boltwoodite. The Canadian Mineralogist, 36 (4) 1069-1075
Burns, Peter C. (2005) U6+ minerals and inorganic compounds: insights into an expanded structural hierarchy of crystal structures. The Canadian Mineralogist, 43 (6) 1839-1894 doi:10.2113/gscanmin.43.6.1839
Frost, Ray L., Čejka, Jiří, Weier, Matt L, Martens, Wayde (2006) Molecular structure of the uranyl silicates—a Raman spectroscopic study. Journal of Raman Spectroscopy, 37 (4). 538-551 doi:10.1002/jrs.1430
Arnold, Thuro, Baumann, Nils (2009) Boltwoodite [K(UO2)(SiO3OH)(H2O)1.5] and compreignacite K2[(UO2)3O2(OH)3]2·7H2O characterized by laser fluorescence spectroscopy. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 71 (5). 1964-1968 doi:10.1016/j.saa.2008.07.029
Shvareva, Tatiana Y., Mazeina, Lena, Gorman-Lewis, Drew, Burns, Peter C., Szymanowski, Jennifer E.S., Fein, Jeremy B., Navrotsky, Alexandra (2011) Thermodynamic characterization of boltwoodite and uranophane: Enthalpy of formation and aqueous solubility. Geochimica et Cosmochimica Acta, 75 (18) 5269-5282 doi:10.1016/j.gca.2011.06.041
Localities for Boltwoodite
Showing 95 localities.
symbol to view information about a locality.
The Locality List
- This locality has map coordinates listed.
- This locality has estimated coordinates.
ⓘ - Click for references and further information on this occurrence.
? - Indicates mineral may be doubtful at this locality.
- Good crystals or important locality for species.
- World class for species or very significant.
(TL) - Type Locality for a valid mineral species.
(FRL) - First Recorded Locality for everything else (eg varieties).
All localities listed without proper references should be considered as questionable.
Argentina | |
| Toubes |
| Honea (1961) |
| Personally collected by Günter Frenz |
| Lucero et al. (1963) |
| Lucero et al. (1963) | |
| Honea (1961) | |
Australia | |
| Honea (1961) |
| Brugger et al. (2003) +1 other reference |
Austria | |
| Blaß et al. (2000) |
Brazil | |
| Pires et al. (2014) |
Canada | |
| G.S.C. Bulletin 330. +1 other reference |
| Miller A. (1980) | |
| Traill (1983) |
| Knipping (1974) |
China | |
| Congjian Pu (1990) | |
| Shen (n.d.) |
| Shen (n.d.) |
Czech Republic | |
| Plášil et al. (2016) |
| Pauliš P. et al. (Kutna Hora, issue 1) |
| Pauliš P. et al. (Kutna Hora, issue 1) |
Egypt | |
| Bahr et al. (2026) |
| Abd El-Moghny et al. (2026) |
France | |
| - (1998) |
Germany | |
| Aufschluss 69/ (7+8) +1 other reference |
| Hans-Jürgen Haas collection |
| Wittern (2001) |
Greece | |
| Michalis Fitros collection (EDS and PXRD analyses) +2 other references |
Hungary | |
| Zsombor Eva |
Iran | |
| Iranmanesh et al. (2018) |
Italy | |
| Savia et al. (2026) |
Japan | |
| K. Watanabe (1976) |
| Watanabe (1976) |
Malawi | |
| Becker |
Mexico | |
| Wong et al. (1999, January) +1 other reference |
Namibia | |
| |
| Palfi et al. (2001) +1 other reference |
| Swakop Uranium | |
| Rio Tinto Group | |
Niger | |
| www.cnncintl.com (2015) |
Norway | |
| Tomas Husdal analytical data |
| Eldjarn (1988) |
| Gunnar Raade (1990) +1 other reference |
Slovakia | |
| Polák Ľ. et al. (2017) |
South Africa | |
| Cairncross et al. (1995) |
Spain | |
| Joan Abella i Creus and Joan Viñals i Olià (2012) +1 other reference |
Sweden | |
| |
| |
| Natural History Museum |
| Natural History Museum | |
| Löfvendahl (1981) |
| |
UK | |
| Elton et al. (1995) |
| Alysson Rowan collection |
| Alysson Rowan collection |
| Tindle (2008) | |
| Handbook of Mineralogy. +1 other reference |
| Miller et al. (1966) +1 other reference | |
| R. S. W. Braithwaite and J. R. Knight (1990) |
Ukraine | |
| Cuney et al. (2012) |
| Dudar et al. (2018) | |
| Dudar et al. (2018) | |
| Cuney et al. (2012) | |
USA | |
| Bollin et al. (1958) +1 other reference |
| Daphne Ross cited in Honea (1961) +1 other reference |
| Daphne Ross cited in Honea (1961) +1 other reference | |
| Anthony et al. (1995) | |
| Austin (1964) +1 other reference | |
| Austin (1964) +1 other reference | |
| Austin (1964) +1 other reference | |
| Scarborough (1981) | |
| 108 +3 other references |
| Eckel et al. (1997) |
| Mineralogical Society of America - ... |
| Gross (1965) +1 other reference |
| Rocks & Minerals. Nov. 1999. |
| Lapham et al. (1965) |
| American Mineralogist: 46: 12-25. +1 other reference |
| - (2005) |
| Smith (1991) | |
| - (2005) |
| Bullock (1981) |
| Collection of Alex Earl | |
| Nickel et al. (1991) |
| Thorne (n.d.) |
| Bullock (1981) |
| Richardson et al. (1993) |
| Bullock (1981) |
| Bullock (1981) | |
| Bullock (1981) |
| Deliens et al. (1991) +3 other references | |
| Bullock (1981) |
| American Mineralogist: 46: 12-25. |
| Vochten et al. (1998) | |
| Honea (1961) |
| Honea (1961) |
Quick NavTopAbout BoltwooditeUnique IdentifiersIMA Classification Classification Mineral SymbolsPhysical Properties Optical Data Chemistry Crystallography Crystal StructureX-Ray Powder DiffractionGeological EnvironmentType Occurrence Other LanguagesRelationshipsCommon AssociatesStrunz-MindatRadioactivityOther InformationInternet Links References Localities Locality List
Goanikontes Claim, Goanikontes, Arandis Constituency, Erongo Region, Namibia