The ideal formulae of two root-names of the hellandite group, i.e. mottanaite and ciprianiite, were originally given as XCa4Y(CeCa)2ZAlTBe2[B4Si4O22]WO2 and XCa4Y[(Th,U)REE]Σ2ZAlT□2[B4Si4O22]W(OH)2. In order to conform to the later introduced dominant-valency nomenclature rule, they have been redefined as XCa4YREE2ZAlT(Be1.5□0.5)[B4Si4O22]WO2 and XCa4Y[(Th,U)Ca]Σ2ZAlT(Be1.5□0.5)Σ2[B4Si4O22]WO2, respectively (IMA-CNMNC vote 18D). Also, the mineral description is provided for the first Fe3+-dominant species of the hellandite group, ferri-mottanaite-(Ce), ideally XCa4YCe2ZFe3+T(Be1.5□0.5)[Si4B4O22]WO2. The type specimen was found in an ejectum collected at Tre Croci (Vetralla, Vico volcanic province, Italy). The empirical formula derived from electron microprobe and LA-ICP-MS analyses, and validated by single-crystal structure refinement is: X(Ca)4Y(Ca0.40REE0.930.13)Σ2.00Z(Al0.23)Σ1.07T(Be1.04Li0.040.92)Σ2.00[Si4.03B3.89O22] (O1.09(OH)0.38F0.53)Σ2.00. Ferri-mottanaite-(Ce) is biaxial (–), with α = 1.748(5), β = 1.762(5), γ = 1.773(5) and 2 V (meas.) = 85.9(5)°, 2 V (calc.) = 82.5°. The unit-cell parameters are a = 19.0548(9), b = 4.7468(2), c = 10.2560(5) Å, β = 110.906(2)°, V = 866.58(7) Å3, Z = 2, space group P2/a. The strongest reflections in the X-ray powder pattern obtained from single-crystal data [d values (in Å), I, (hkl)] are: 2.648, 100, (013,13); 2.857, 50, (411); 1.904, 48, (023,23,21); 2.919, 44, (212); 3.086, 44, (12); 3.246, 43, (10); 3.453, 36, (12); 4.745, 33, (010).
News from the hellandite group: the redefinition of mottanaite and ciprianiite and the new mineral description of ferri-mottanaite-(Ce), the first Fe3+-dominant hellandite
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Roberta Oberti, Antonio Langone, Massimo Boiocchi, Ezio Bernabè, Frank C. Hawthorne; News from the hellandite group: the redefinition of mottanaite and ciprianiite and the new mineral description of ferri-mottanaite-(Ce), the first Fe3+-dominant hellandite. European Journal of Mineralogy doi: https://doi.org/10.1127/ejm/2019/0031-2858
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