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 ^XCa₄^Y(CeCa)₂^ZAl^TBe₂[B₄Si₄O₂₂]^WO₂ and ^XCa₄^Y[(Th,U)REE]_Σ2^ZAl^T□₂[B₄Si₄O₂₂]^W(OH)₂. In order to conform to the later introduced dominant-valency nomenclature rule, they have been redefined as ^XCa₄^YREE₂^ZAl^T(Be_1.5□_0.5)[B₄Si₄O₂₂]^WO₂ and ^XCa₄^Y[(Th,U)Ca]_Σ2^ZAl^T(Be_1.5□_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 ^XCa₄^YCe₂^ZFe^3+T(Be_1.5□_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)₄^Y(Ca_0.40REE_0.93$(Th,U)0.544+$_0.13)_Σ2.00^Z(⁠$Fe0.503+$Al_0.23$Mn0.173+Ti0.174+$⁠)_Σ1.07^T(Be_1.04Li_0.04_0.92)_Σ2.00[Si_4.03B_3.89O₂₂] (O_1.09(OH)_0.38F_0.53)_Σ2.00. Ferri-mottanaite-(Ce) is biaxial (–), with α = 1.748(5), β = 1.762(5), γ = 1.773(5) and 2V (meas.) = 85.9(5)°, 2V (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) ų, 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,$4¯$13); 2.857, 50, (411); 1.904, 48, (023,$4¯$23,$6¯$21); 2.919, 44, (212); 3.086, 44, (⁠$4¯$12); 3.246, 43, (⁠$4¯$10); 3.453, 36, (⁠$2¯$12); 4.745, 33, (010).