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Frank Wigglesworth Clarke
(Na,Ca,Pb)(UO2)O(OH) · 0-1H2O
Dark red-brown, dark ...
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

Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"

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 URL:
Please feel free to link to this page.

Type Occurrence of Clarkeite

Year of Discovery:

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)    

Crystallography of Clarkeite

Crystal System:
Cell Parameters:
a = 3.954Å, c = 17.660Å
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.

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

(Na,Ca,Pb)(UO2)O(OH) · 0-1H2O
Essential elements:
All elements listed in formula:

Relationship of Clarkeite to other Species

4.GC.10Umohoite(UO2)MoO4 · 2H2O
4.GC.15SpriggitePb3(UO2)6O8(OH)2 · 3H2O
7.16.2Schoepite(UO2)8O2(OH)12 · 12H2O
7.16.3ParaschoepiteUO3 · 2H2O
7.16.4MetaschoepiteUO3 · 1-2H2O
7.16.5MetastudtiteUO4 · 2H2O
7.16.6Studtite[(UO2)(O2)(H2O)2] · H2O
7.16.7IanthiniteU4+(UO2)5O7 · 10H2O
7.16.8CompreignaciteK2(UO2)6O4(OH)6 · 7H2O
7.16.11BecquereliteCa(UO2)6O4(OH)6 · 8H2O
7.16.12RameauiteK2Ca(UO2)6(OH)16 · H2O
7.16.13AgrinieriteK2(Ca,Sr)[(UO2)3O3(OH)2]2 · 5H2O
7.16.14ProtasiteBa(UO2)3O3(OH)2 · 3H2O
7.16.15BillietiteBa(UO2)6O4(OH)6 · 4-8H2O
7.16.16BauranoiteBa(UO2)2(OH)6 · 1-2H2O
7.16.17Metacalciouranoite(Ca,Ba,Pb,K2)U2O7 · 2H2O
7.16.18Calciouranoite(Ca,Ba,Pb)U2O7 · 5H2O
7.16.24FourmarieritePb(UO2)4O3(OH)4 · 4H2O
7.16.25Richetite(Fe3+,Mg)Pb 8.6(UO2)36O36(OH)24•41H2O
7.16.26CuritePb3(UO2)8O8(OH)6 · 3H2O
7.16.27MasuyitePb(UO2)3O3(OH)2 · 3H2O
7.16.28VandendriesscheitePbU7O22 · 12H2O
7.16.29MetavandendriesscheitePbU7O22 · nH2O n < 12
7.16.30SayritePb2(UO2)5O6(OH)2 · 4H2O
7.16.31Wölsendorfite(Pb,Ca)U2O7 · 2H2O

Other Names for Clarkeite

Name in Other Languages:
Simplified Chinese:水钠铀矿

Other Information

Health Risks:
No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.

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

Localities for Clarkeite

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
  • 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
          • 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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