Clarkeite
A valid IMA mineral species - grandfathered
This page is currently not sponsored. Click here to sponsor this page.
About Clarkeite
Frank Wigglesworth Clarke
Formula:
(Na,Ca,Pb)(UO2)O(OH) · 0-1H2O
Colour:
Dark red-brown, dark brown
Lustre:
Resinous, Waxy, Greasy
Hardness:
4 - 4½
Specific Gravity:
6.29 - 6.39
Crystal System:
Trigonal
Name:
Named in 1931 by Clarence S. Ross, Edward P. Henderson, and Eugene Waldemar Posnjak in honor of Frank Wigglesworth Clarke [March 19, 1847 Boston, Massachusetts, USA - May 23, 1931 Washington, DC, USA], American mineral chemist, and former Chief Chemist of the U.S. Geological Survey. Clarke studied the composition of the Earth's crust and the unit of measure, the clarke, was named as a reference to the local enrichment of a chemical component relative to an average value for the chemistry of the crust.
Clarkeite is a waxy to resinous dark brown replacement of uraninite. It is very uncommon, but is frequently misidentified with brown gummite that is dominated by schoepite-group minerals. Frondel (1956) warned: "Clarkeite is best identified by its X-ray powder pattern. It is indistinguishable from the dark brown types of oxidized uraninite ... and from dense brown types of fourmarierite and vandendriesscheite."
Unique Identifiers
Mindat ID:
1059
Long-form identifier:
mindat:1:1:1059:3
IMA Classification of Clarkeite
Approved, 'Grandfathered' (first described prior to 1959)
IMA Formula:
Na(U6+O2)O(OH)·nH2O
First published:
1931
Classification of Clarkeite
4.GC.05
4 : OXIDES (Hydroxides, V[5,6] vanadates, arsenites, antimonites, bismuthites, sulfites, selenites, tellurites, iodates)
G : Uranyl Hydroxides
C : With additional cations; with mainly UO2(O,OH)6 hexagonal polyhedra
4 : OXIDES (Hydroxides, V[5,6] vanadates, arsenites, antimonites, bismuthites, sulfites, selenites, tellurites, iodates)
G : Uranyl Hydroxides
C : With additional cations; with mainly UO2(O,OH)6 hexagonal polyhedra
5.4.1.1
5 : OXIDES CONTAINING URANIUM OR THORIUM
4 : AX2O7·xH2O
5 : OXIDES CONTAINING URANIUM OR THORIUM
4 : AX2O7·xH2O
7.16.32
7 : Oxides and Hydroxides
16 : Oxides of U
7 : Oxides and Hydroxides
16 : Oxides 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 |
|---|---|---|
| Cke | 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 Clarkeite
Resinous, Waxy, Greasy
Transparency:
Translucent
Colour:
Dark red-brown, dark brown
Streak:
Yellow-brown
Hardness:
4 - 4½ on Mohs scale
Tenacity:
Brittle
Cleavage:
None Observed
Fracture:
Conchoidal, Sub-Conchoidal
Density:
6.29 - 6.39 g/cm3 (Measured) 6.74 g/cm3 (Calculated)
Optical Data of Clarkeite
Type:
Biaxial (-)
RI values:
nα = 1.997 nβ = 2.098 nγ = 2.108
2V:
Measured: 30° to 50°, Calculated: 32°
Birefringence:
0.111
Max. Birefringence:
δ = 0.111
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.
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.
Dispersion:
relatively weak
Pleochroism:
Weak
Comments:
Orange and slightly deeper orange
Chemistry of Clarkeite
Mindat Formula:
(Na,Ca,Pb)(UO2)O(OH) · 0-1H2O
Element Weights:
Crystallography of Clarkeite
Crystal System:
Trigonal
Class (H-M):
3m(32/m) - Hexagonal Scalenohedral
Space Group:
R3m
Setting:
R3m
Cell Parameters:
a = 3.954(1) Å, c = 17.660(3) Å
Ratio:
a:c = 1 : 4.466
Unit Cell V:
239.11 ų (Calculated from Unit Cell)
Z:
3
Morphology:
Massive, dense.
Crystal Structure
Load
Unit Cell | Unit Cell Packed
2x2x2 | 3x3x3 | 4x4x4
Unit Cell | Unit Cell Packed
2x2x2 | 3x3x3 | 4x4x4
Show
Big Balls | Small Balls | Just Balls | Spacefill
Polyhedra Off | Si Polyhedra | All Polyhedra
Remove metal-metal sticks
Big Balls | Small Balls | Just Balls | Spacefill
Polyhedra Off | Si Polyhedra | All Polyhedra
Remove metal-metal sticks
Display Options
Black Background | White Background
Perspective On | Perspective Off
2D | Stereo | Red-Blue | Red-Cyan
Black Background | White Background
Perspective On | Perspective Off
2D | Stereo | Red-Blue | Red-Cyan
View
CIF File Best | x | y | z | a | b | c
CIF File Best | x | y | z | a | b | c
Rotation
Stop | Start
Stop | Start
Labels
Console Off | On | Grey | Yellow
Console Off | On | Grey | Yellow
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) |
|---|---|---|---|---|---|---|---|
| 0001902 | Clarkeite | Finch R J, Ewing R C (1997) Clarkeite: New chemical and structural data American Mineralogist 82 607-619 |  | 1997 | 0 | 293 |
CIF Raw Data - click here to close
X-Ray Powder Diffraction
Powder Diffraction Data:
| d-spacing | Intensity |
|---|---|
| 5.903 Å | (64) |
| 3.365 Å | (56) |
| 3.195 Å | (100) |
| 2.945 Å | (28) |
| 2.707 Å | (34) |
| 1.977 Å | (33) |
| 1.8742 Å | (19) |
| 1.8554 Å | (19) |
| 1.6807 Å | (14) |
| 1.6410 Å | (23) |
Comments:
AM 82, p. 615; JCPDS 8-315 has extraneous and differing d-values.
Geological Environment
Paragenetic Mode(s):
| Paragenetic Mode | Earliest Age (Ga) |
|---|---|
| Stage 7: Great Oxidation Event | <2.4 |
| 47a : [Near-surface hydration of prior minerals] | |
| 47f : [Uranyl (U⁶⁺) minerals] |
Type Occurrence of Clarkeite
General Appearance of Type Material:
Massive, without cleavage
Place of Conservation of Type Material:
National Museum of Natural History, Washington, D.C., USA, 96510, R6607.
Geological Setting of Type Material:
Uranium bearing pegmatite
Associated Minerals at Type Locality:
Synonyms of Clarkeite
Other Language Names for Clarkeite
Common Associates
Associations Based on Photo Data:
| 30 photos of Clarkeite associated with Uraninite | UO2 |
| 27 photos of Clarkeite associated with Uranophane | Ca(UO2)2(SiO3OH)2 · 5H2O |
| 23 photos of Clarkeite associated with 'Gummite' | |
| 21 photos of Clarkeite associated with Kasolite | Pb(UO2)(SiO4) · H2O |
| 20 photos of Clarkeite associated with Fourmarierite | Pb(UO2)4O3(OH)4 · 4H2O |
| 12 photos of Clarkeite associated with Muscovite | KAl2(AlSi3O10)(OH)2 |
| 10 photos of Clarkeite associated with Rutherfordine | (UO2)CO3 |
| 9 photos of Clarkeite associated with 'Cyrtolite' | Zr[(SiO4),(OH)4] |
| 3 photos of Clarkeite associated with Autunite | Ca(UO2)2(PO4)2 · 10-12H2O |
| 2 photos of Clarkeite associated with Albite | Na(AlSi3O8) |
Related Minerals - Strunz-mindat Grouping
| 4.GC. | Bobfinchite | Na[(UO2)8O3(OH)11] · 10H2O |
| 4.GC.10 | Umohoite | (UO2)MoO4 · 2H2O |
| 4.GC.15 | Spriggite | Pb3(UO2)6O8(OH)2 · 3H2O |
Radioactivity
Radioactivity:
| Element | % Content | Activity (Bq/kg) | Radiation Type |
|---|---|---|---|
| Uranium (U) | 73.0096% | 18,252,400 | α, β, γ |
| Thorium (Th) | 0.0000% | 0 | α, β, γ |
| Potassium (K) | 0.0000% | 0 | 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
Fluorescence of Clarkeite
none
Other Information
Notes:
Readily dissolves in mineral acids and hydrochloric acid. Can be dissolved in very dilute acid (1 part acid to 4 parts water).
Health Risks:
radioactive
Internet Links for Clarkeite
mindat.org URL:
https://www.mindat.org/min-1059.html
Please feel free to link to this page.
Please feel free to link to this page.
Search Engines:
External Links:
Mineral Dealers:
References for Clarkeite
Reference List:
Ross, Clarence S., Henderson, E. P., Posnjak, and E. (1931) Clarkeite, a new uranium mineral. American Mineralogist, 16 (5) 213-220
Frondel, Clifford, Meyrowitz, Robert (1956) Studies of uranium minerals (XIX): Rutherfordine, diderichite, and clarkeite. American Mineralogist, 41 (1-2) 127-133
Frondel, Clifford (1958) Systematic mineralogy of uranium and thorium. Bulletin 1064. US Geological Survey doi:10.3133/b1064
Finch, Robert J.; Ewing, Rodney C. (1997) Clarkeite: New chemical and structural data. American Mineralogist, 82 (5). 607-619 doi:10.2138/am-1997-5-620
Kubatko, K.-A., Helean, K., Navrotsky, A., Burns, P. C. (2006) Thermodynamics of uranyl minerals: Enthalpies of formation of uranyl oxide hydrates. American Mineralogist, 91 (4) 658-666 doi:10.2138/am.2006.1856
Gorman-Lewis, Drew, Fein, Jeremy B., Burns, Peter C., Szymanowski, Jennifer E.S., Converse, Jenalee (2008) Solubility measurements of the uranyl oxide hydrate phases metaschoepite, compreignacite, Na–compreignacite, becquerelite, and clarkeite. The Journal of Chemical Thermodynamics, 40 (6) 980-990 doi:10.1016/j.jct.2008.02.006
Localities for Clarkeite
Showing 32 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 | |
| Angelelli (1984) |
Brazil | |
| Pires et al. (2014) |
Canada | |
| Chatterjee (1977) |
Czech Republic | |
| Pauliš P. et al. (Kutna Hora, issue 1) |
Egypt | |
| Abd El-Moghny et al. (2026) |
| Abd El-Moghny et al. (2026) |
Hungary | |
| Koch (1985) |
India | |
| Singh et al. (2015) |
| Dhana Raju (2019) |
| American Mineralogist: 41: 127-133 |
Norway | |
| Neumann (1985) |
| Sverdrup (1959) |
| Neumann (1985) +1 other reference |
| Husdal (2023) |
Romania | |
| Hîrtopanu P. et al. (2004) |
Ukraine | |
| Liventseva (n.d.) |
USA | |
| - (2008) |
| King et al. (1994) |
| King et al. (1994) |
| Frondel (1956) +1 other reference |
| Not found at this locality: Frondel (1956) +1 other reference |
| Smith (2005) |
| Eric Quinter collection |
| Rocks and Minerals |
| Roger Howell Collection | |
| Ross et al. (1931) +1 other reference |
| Rocks and Minerals | |
| Smithsonian Institution Mineral ... |
| Rocks and Minerals |
| www.excaliburmineral.com | |
| Finch et al. (1997) +1 other reference |
| Rocks and Minerals |
Quick NavTopAbout ClarkeiteUnique IdentifiersIMA Classification Classification Mineral SymbolsPhysical Properties Optical Data Chemistry Crystallography Crystal StructureX-Ray Powder DiffractionGeological EnvironmentType Occurrence SynonymsOther LanguagesCommon AssociatesStrunz-MindatRadioactivityFluorescence Other InformationInternet Links References Localities Locality List
Fanny Gouge Mine, Micaville, Celo, Yancey County, North Carolina, USA