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

The ideal formulae of two root-names of the hellandite group, i.e. mottanaite and ciprianiite, were originally given as Ca-X(4)Y(CeCa)(2)(AlBe2)-Al-Z-Be-T[B4Si4O22]O-W(2) and Ca-X(4)Y(Th,U)REE](Sigma 2)Al-Z(T) square(2)[B4Si4O22](W)(OH)(2). In order to conform to the later introduced dominant-valency nomenclature rule, they have been redefined as (Ca4REE2AlT)-Ca-X-R-Y-Al-Z(Be-1.5 square(0.5))[B4Si4O22]O-W(2) and Ca-X(4)Y [Th,U)Ca](Sigma 2)Al-Z(T) (Be-0.5 square(0.5))(Sigma 2)[B4Si4O22](W)(OH)(2), 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 Ca-X(4)Y (Ce2Fe3+T)-Fe-Z (Be-1.5 square(0.5)) [Si4B4O22]O-W(2). 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)(4)(Y)(Ca0.40REE0.93 (Th,U)(0.54)(4+)square(0.13))(Sigma 2.00) (Z)(Fe0.503+Al0.23Mn0.173+Ti0.174+)(Sigma 1.07)(T)(B1.04Li0.04 square(0.92))=Sigma(2.00)[Si4.03B3.89O22] (O-1.09(OH)(0.38)F-0.53)(Sigma 2.00.) Ferri-mottanaite-(Ce) is biaxial (-), with alpha = 1.748(5), beta = 1.762(5), gamma = 1.773(5) and 2V (meas.) = 85.9(5)degrees, 2V (calc.) = 82.5 degrees. The unit-cell parameters are a = 19.0548(9), b = 4.7468(2), c = 10.2560(5) angstrom, beta = 110.906(2)degrees, V = 866.58(7) angstrom(3) , Z = 2, space group P2/a. The strongest reflections in the X-ray powder pattern obtained from single-crystal data [d values (in angstrom), I, (hkl)] are: 2.648, 100, (013,(4) over bar 13); 2.857, 50, (411); 1.904, 48, (023,(4) over bar 23,621); 2.919, 44, (212); 3.086, 44, ((4) over bar 12); 3.246, 43, ((4) over bar 10); 3.453, 36, ((2) over bar 12); 4.745, 33, (010).