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Frank Wigglesworth Clarke
Na(UO2)O(OH) · nH2O
Dark red-brown, dark ...
Resinous, Waxy, Greasy
4 - 4½
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."

Classification of Clarkeite

Approved, 'Grandfathered' (first described prior to 1959)

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 : AX2O7·xH2O

7 : Oxides and Hydroxides
16 : Oxides of U

Physical Properties of Clarkeite

Resinous, Waxy, Greasy
Diaphaneity (Transparency):
Dark red-brown, dark brown
Hardness (Mohs):
4 - 4½
Conchoidal, Sub-Conchoidal
6.39 g/cm3 (Measured)    

Optical Data of Clarkeite

Biaxial (-)
RI values:
nα = 1.997 nβ = 2.098 nγ = 2.108
Measured: 30° to 50°, Calculated: 32°
Max Birefringence:
δ = 0.111
Image shows birefringence interference colour range (at 30µm thickness) and does not take into account mineral colouration.
Surface Relief:
Very High
relatively weak

Chemical Properties of Clarkeite

IMA Formula:
Na(UO2)O(OH) · nH2O
Elements listed in formula:

Crystallography of Clarkeite

Crystal System:
Cell Parameters:
a = 3.954 Å, c = 17.660 Å
a:c = 1 : 4.466
Unit Cell Volume:
V 239.11 ų (Calculated from Unit Cell)
Massive, dense.
X-Ray Powder Diffraction Data:
5.903 (64)
3.365 (56)
3.195 (100)
2.945 (28)
2.707 (34)
1.977 (33)
1.8742 (19)
1.8554 (19)
1.6807 (14)
1.6410 (23)
AM 82, p. 615; JCPDS 8-315 has extraneous and differing d-values.

Type Occurrence of Clarkeite

Relationship of Clarkeite to other Species

4.GC.10Umohoite(UO2)Mo6+O4 · 2H2O
4.GC.15SpriggitePb3(UO2)6O8(OH)2 · 3H2O
7.16.2Schoepite(UO2)8O2(OH)12 · 12H2O
7.16.3ParaschoepiteUO3 · (2-x)H2O
7.16.4Metaschoepite(UO2)8O2(OH)12 · 10H2O
7.16.6Studtite(UO2)O2(H2O)2 · 2H2O
7.16.7IanthiniteU24+(UO2)4O6(OH)4 · 9H2O
7.16.8CompreignaciteK2(UO2)6O4(OH)6 · 7H2O
7.16.10RoubaultiteCu2O2(UO2)3(CO3)2(OH)2 · 4H2O
7.16.11BecquereliteCa(UO2)6O4(OH)6 · 8H2O
7.16.12RameauiteK2Ca(UO2)6O6(OH)4 · 6H2O
7.16.13AgrinieriteK2Ca[(UO2)3O3(OH)2]2 · 5H2O
7.16.14ProtasiteBa(UO2)3O3(OH)2 · 3H2O
7.16.15BillietiteBa(UO2)6O4(OH)6 · 8H2O
7.16.16BauranoiteBaU2O7 · 4-5H2O
7.16.17Metacalciouranoite(Ca,Na,Ba)U2O7 · 2H2O
7.16.18Calciouranoite(Ca,Ba,Pb,K,Na)U2O7 · 5H2O
7.16.24FourmarieritePb1-xO3-2x(UO2)4(OH)4+2x · 4H2O
7.16.25Richetite(Fe3+,Mg)xPb2+8.6(UO2)36O36(OH)24 · 41H2O
7.16.26CuritePb3+x[(UO2)4O4+x(OH)3-x]2 · 2H2O
7.16.27MasuyitePb(UO2)3O3(OH)2 · 3H2O
7.16.28VandendriesscheitePb1.6(UO2)10O6(OH)11 · 11H2O
7.16.29MetavandendriesscheitePbU7O22 · nH2O
7.16.30SayritePb2(UO2)5O6(OH)2 · 4H2O
7.16.31WölsendorfitePb7(UO2)14O19(OH)4 · 12H2O

Other Names for Clarkeite

Name in Other Languages:
Simplified Chinese:水钠铀矿

Other Information

Health Risks:

References for Clarkeite

Reference List:
Ross, Henderson, and Posnjak (1931), American Mineralogist: 16: 213.

Palache, Charles, Harry Berman & Clifford Frondel (1944), The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana Yale University 1837-1892, Volume I: Elements, Sulfides, Sulfosalts, Oxides. John Wiley and Sons, Inc., New York. 7th edition, revised and enlarged: 624-625.

Frondel, Clifford, 1956, Systematic Mineralogy of Uranium and Thorium, USGS Bulletin 1064, pp. 400.

Finch, Robert J. and Ewing, Rodney C., 1997, Clarkeite: New Chemical and Structural Data, American Mineralogist, 82: 607-619.

Finch, R.J. and R.C. Ewing (1997), Clarkeite: New chemical and structural data: American Mineralogist: 82: 607-619.

Kubatko, Karrie-Ann, Katheryn Helean, Alexandra Navrotsky, and Peter C. Burns (2006), Thermodynamics of uranyl minerals: Enthalpies of formation of uranyl oxide hydrates: American Mineralogist: 91: 658-666.

Internet Links for Clarkeite URL:
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The following Clarkeite specimens are currently listed for sale on

Localities for Clarkeite

This map shows a selection of localities that have latitude and longitude coordinates recorded. Click on the symbol to view information about a locality. The symbol next to localities in the list can be used to jump to that position on the map.
(TL) indicates type locality for a valid mineral species. (FRL) indicates first recorded locality for everything else. ? indicates mineral may be doubtful at this locality. All other localities listed without reference should be considered as uncertain and unproven until references can be found.
  • Nova Scotia
    • Cumberland Co.
A.K. Chatterjee, "Uranium Mineralization at McLean Point, Cumberland County", Nova Scotia Dept. Mines Report 77-1, 1977.
Czech Republic
  • Bohemia (Böhmen; Boehmen)
    • Plzeň Region
      • Planá u Mariánských Lázní (Plan)
Pauliš P., Kopecký S., Černý P. 2007: Uranové minerály České Republiky a jejich naleziště. 1. část. (Kutna Hora, issue 1)
  • Baranya Co.
    • Mecsek Mts
Koch: Minerals of Hungary, 1985
  • Rajasthan (Rajputana)
    • Ajmer Division
American Mineralogist: 41: 127-133
  • Aust-Agder
Neumann, H. (1985): Norges Mineraler. Norges geologiske Undersøkelse Skrifter 68, p 90
  • Buskerud
    • Krødsherad
      • Krøderen Lake
Neumann, H. (1985): Norges Mineraler. Norges geologiske Undersøkelse Skrifter 68, pp. 89-90
  • Vest-Agder
    • Lindesnes
      • Vigeland
Sverdrup, T. L. (1959): The pegmatite dyke at Rømteland. Norges Geologiske Undersøkelse 211 (Årbok 1959): 124-196
  • Suceava Co.
    • Suceava district
Hîrtopanu P., Udubaşa G., Scott P., Hârtopanu I. (2004) Uranium sulphide mineralisation in the Bistriţa Mts., East Carpathians, Romania. Romanian Journal of Mineral Deposits 81:108-111
  • Alaska
    • Yukon-Koyukuk Borough
      • Sheenjek District
U.S. Geological Survey, 2008, Alaska Resource Data File (ARDF): U.S. Geological Survey Open-File Report 2008-1225.
  • Maine
    • Oxford Co.
      • Newry
Not found according to King, V. T., 1994, Mineralogy of Maine, Descriptive Mineralogy, v. 1
      • Stoneham
Not found according to King & Foord, 1994. Mineralogy of Maine, V. 1.
  • New Hampshire
    • Grafton Co.
      • Grafton
Discredited by Clifford Frondel (1956) Mineral Composition of Gummite, American Mineralogist 41:539-568.; Rocks & Minerals (2005) 80:242-261 New Hampshire Mineral Locality Index
      • Groton
Not found at this locality: Frondel (1956); Foord et al. (1997), Korzeb et al (1997).
    • Rockingham Co.
      • Raymond
Rocks & Minerals (2005) 80:242-261 New Hampshire Mineral Locality Index
  • North Carolina
    • Mitchell Co.
Rocks and Minerals, Sep-Oct,2004
Roger Howell Collection
Rocks and Minerals,Sep-Oct,2004
      • Spruce Pine District
        • Penland
Ross, Henderson, and Posnjak (1931), American Mineralogist: 16: 213.; Arthur Roe and John S. White, Jr. (1976) A Catalog of the Type Specimens in the Mineral Collection, National Museum of Natural History
        • Spruce Pine
Smithsonian Institution Mineral Reference Collection No.R13655-00
          • Greasy Creek Township
Rocks and Minerals,Sep-Oct,2004
Rocks and Minerals,Sep-Oct,2004
    • Yancey Co.
      • Spruce Pine District
        • Celo
          • Micaville
U.S. Geological Survey, 2005, Mineral Resources Data System: U.S. Geological Survey, Reston, Virginia.; American Mineralogist, Volume 82, pages 607–619, 1997
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