Bicapite, KNa2Mg2(H2PV5+14 O42)·25H2O, a new polyoxometalate mineral with a bicapped Keggin anion from the Pickett Corral mine, Montrose County, Colorado, U.S.A
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Anthony R. Kampf
,
John M. Hughes
Abstract
Bicapite, KNa2Mg2(H2PV145+ O42)·25H2O, is a new mineral species (IMA2018-048) discovered at the Pickett Corral mine, Montrose County, Colorado, U.S.A. Bicapite occurs as square tablets up to about 0.2 mm on edge on montroseite-corvusite-bearing sandstone. Crystals are dark red-brown, often appearing black. The streak is orange, and the luster is vitreous. Bicapite is brittle, has a Mohs hardness of 1½, and displays one excellent cleavage on {100}. The measured density is 2.44(2) g/cm3. Bicapite is uniaxial (+), w = 1.785(5), e ≈ 1.81 (white light); pleochroism is red-brown; E > O, slight. The electron probe microanalysis and results of the crystal structure determination provided the empirical formula (based on 67 O apfu) (K1.23Na2.23Mg1.48)∑4.94[H2.51P1.02(V13.915+ Mo0.076+ )∑13.98O42]·25H2O. Bicapite is tetragonal, I4/m, with a = 11.5446(12) Å, c = 20.5460(14) Å, V = 2738.3(6) Å3, and Z = 2. The strongest four lines in the diffraction pattern are [d in Å (I) (hkl)]: 10.14 (100) (002,101); 2.978 (29) (134,206); 2.809 (11) (305); and 2.583 (11) (420,008). The atomic arrangement of bicapite was solved and refined to R1 = 0.0465 for 1008 independent reflections with I > 2sI. The structural unit is a [H2PV125+ O40(V5+O)2]7– heteropolyanion composed of 12 distorted VO6 octahedra surrounding a central PO4 tetrahedron and capped on opposite sides by two VO5 square pyramids; the structural unit is a modification of the a-isomer of the Keggin anion, [XM12O40]n-. Charge balance in the structure is maintained by the [KNa2Mg2(H2O)25]7+ interstitial complex. The name bicapite is in recognition of this being the only known mineral with a structure based on a bicapped Keggin anion. The discovery of bicapite and numerous other natural polyoxometalate compounds in the Colorado Plateau uranium/ vanadium deposits make that the most productive region found to date for naturally occurring polyoxometalate compounds.
Acknowledgments
The manuscript was improved by reviews by Michael Schindler and an anonymous review. Editorial handling by Fernando Colombo is greatly appreciated.
Funding
This study was funded, in part, by the John Jago Trelawney Endowment to the Mineral Sciences Department of the Natural History Museum of Los Angeles County and by grant NSF-MRI 1039436 from the National Science Foundation to J.M.H. B.P.N. thanks the Northern California Mineralogical Association for partial support for electron microprobe analyses of new minerals from the Colorado Plateau.
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Articles in the same Issue
- Invited Centennial Review
- High-pressure minerals
- Crossroads in Earth and Planetary Materials
- Investigation of the kieserite–szomolnokite solid-solution series, (Mg,Fe)SO4·H2O, with relevance to Mars: Crystal chemistry, FTIR, and Raman spectroscopy under ambient and martian temperature conditions
- Intra-eruptive trachyte-phonolite transition: Natural evidence and experimental constraints on the role of crystal mushes
- Geochemistry of phosphorus and the behavior of apatite during crustal anatexis: Insights from melt inclusions and nanogranitoids
- Static compression of Fe4N to 77 GPa and its implications for nitrogen storage in the deep Earth
- A comparison between the stability fields of a Cl-rich scapolite and the end-member marialite
- Electrical conductivity studies on silica phases and the effects of phase transformation
- REE-, Sr-, Ca-aluminum-phosphate-sulfate minerals of the alunite supergroup and their role as hosts for radionuclides
- Experimental quantification of the Fe-valence state at amosite-asbestos boundaries using acSTEM dual-electron energy-loss spectroscopy
- Thermoelasticity, cation exchange, and deprotonation in Fe-rich holmquistite: Toward a crystal-chemical model for the high-temperature behavior of orthorhombic amphiboles
- Chemical and textural relations of britholite- and apatite-group minerals from hydrothermal REE mineralization at the Rodeo de los Molles deposit, Central Argentina
- Bicapite, KNa2Mg2(H2PV5+14 O42)·25H2O, a new polyoxometalate mineral with a bicapped Keggin anion from the Pickett Corral mine, Montrose County, Colorado, U.S.A
- Solubility of carbon and nitrogen in a sulfur-bearing iron melt: Constraints for siderophile behavior at upper mantle conditions
- New Mineral Names
- Editors’ note
Articles in the same Issue
- Invited Centennial Review
- High-pressure minerals
- Crossroads in Earth and Planetary Materials
- Investigation of the kieserite–szomolnokite solid-solution series, (Mg,Fe)SO4·H2O, with relevance to Mars: Crystal chemistry, FTIR, and Raman spectroscopy under ambient and martian temperature conditions
- Intra-eruptive trachyte-phonolite transition: Natural evidence and experimental constraints on the role of crystal mushes
- Geochemistry of phosphorus and the behavior of apatite during crustal anatexis: Insights from melt inclusions and nanogranitoids
- Static compression of Fe4N to 77 GPa and its implications for nitrogen storage in the deep Earth
- A comparison between the stability fields of a Cl-rich scapolite and the end-member marialite
- Electrical conductivity studies on silica phases and the effects of phase transformation
- REE-, Sr-, Ca-aluminum-phosphate-sulfate minerals of the alunite supergroup and their role as hosts for radionuclides
- Experimental quantification of the Fe-valence state at amosite-asbestos boundaries using acSTEM dual-electron energy-loss spectroscopy
- Thermoelasticity, cation exchange, and deprotonation in Fe-rich holmquistite: Toward a crystal-chemical model for the high-temperature behavior of orthorhombic amphiboles
- Chemical and textural relations of britholite- and apatite-group minerals from hydrothermal REE mineralization at the Rodeo de los Molles deposit, Central Argentina
- Bicapite, KNa2Mg2(H2PV5+14 O42)·25H2O, a new polyoxometalate mineral with a bicapped Keggin anion from the Pickett Corral mine, Montrose County, Colorado, U.S.A
- Solubility of carbon and nitrogen in a sulfur-bearing iron melt: Constraints for siderophile behavior at upper mantle conditions
- New Mineral Names
- Editors’ note
