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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2017: 2.645

CiteScore 2018: 2.55

SCImago Journal Rank (SJR) 2018: 1.355
Source Normalized Impact per Paper (SNIP) 2018: 1.103

Online
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1945-3027
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Volume 100, Issue 1

Issues

Nuragheite, Th(MoO4)2·H2O, The Second Natural Thorium Molybdate And Its Relationships To Ichnusaite And Synthetic Th(MoO4)2†

Paolo Orlandi
  • Dipartimento di Scienze della Terra, Università di Pisa, via S. Maria 53, I-56126 Pisa, Italy
  • Istituto di Geoscienze e Georisorse, CNR, Via Moruzzi 1, I-56124 Pisa, Italy
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/ Cristian Biagioni
  • Corresponding author
  • Dipartimento di Scienze della Terra, Università di Pisa, via S. Maria 53, I-56126 Pisa, Italy
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/ Luca Bindi
  • Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via G. La Pira, 4, I-50121 Firenze, Italy
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  • De Gruyter OnlineGoogle Scholar
/ Stefano Merlino
  • Dipartimento di Scienze della Terra, Università di Pisa, via S. Maria 53, I-56126 Pisa, Italy
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Published Online: 2015-01-10 | DOI: https://doi.org/10.2138/am-2015-5024

Abstract

The new mineral species nuragheite, Th(MoO4)2·H2O, has been discovered in the Mo-Bi mineralization of Su Seinargiu, Sarroch, Cagliari, Sardinia, Italy. It occurs as colorless thin {100} tabular crystals, up to 200 μm in length, associated with muscovite, xenotime-(Y), and ichnusaite, Th(MoO4)2·3H2O. Luster is pearly to adamantine; nuragheite is brittle, with a perfect (100) cleavage. Owing to the very small amount of available material and its intimate association with ichnusaite, density and optical properties were not measured. Electron microprobe analysis gave (wt% = mean of six spot analyses): MoO3 49.38, ThO2 45.39, H2Ocalc 3.09, total 97.86. On the basis of eight O atoms per formula unit and assuming one H2O group, in agreement with the crystal structure data, the chemical formula of nuragheite is Th1.00Mo2.00O8·H2O. Main diffraction lines, corresponding to multiple hkl indices, are [d in Å (relative visual intensity)]: 5.28 (m), 5.20 (m), 5.04 (m), 4.756 (m), 3.688 (m), 3.546 (vs), 3.177 (s), 3.024 (m). The crystal structure study gives a monoclinic unit cell, space group P21/c, with a = 7.358(2), b = 10.544(3), c = 9.489(2) Å, β = 91.88(2)°, V = 735.8(2) Å3, Z = 4. The crystal structure has been solved and refined to a final R1 = 0.078 on the basis of 1342 “observed” reflections [Fo > 4σ(Fo)]. It consists of (100) layers formed by ninefold-coordinated Th-centered polyhedra and Mo-centered tetrahedra. Its crystal structure is discussed in relation to that of ichnusaite and that of synthetic orthorhombic Th(MoO4)2. The relationship between the progressive loss of water in the interlayer and the layer topology passing from ichnusaite through nuragheite to synthetic (ThMoO4)2 is examined. Nuragheite, the second thorium molybdate reported so far in nature, adds new data to the understanding of the crystal chemistry of actinide molybdates potentially forming during the alteration of spent nuclear fuel and influencing the release of radionuclides under repository conditions.

Keywords : Nuragheite; new mineral species; molybdate; thorium; crystal structure; OD structure; Su Seinargiu; Sardinia; Italy

About the article

Received: 2014-04-27

Accepted: 2014-07-02

Published Online: 2015-01-10

Published in Print: 2015-01-01


Citation Information: American Mineralogist, Volume 100, Issue 1, Pages 267–273, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2015-5024.

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© 2015 by Walter de Gruyter Berlin/Boston.

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