Mianningite, (☐,Pb,Ce,Na) (U⁴⁺,Mn,U⁶⁺) Fe³⁺₂(Ti,Fe³⁺)₁₈O₃₈, a new member of the crichtonite group from Maoniuping REE deposit, Mianning county, southwest Sichuan, China

Mianningite (IMA 2014-072), ideally (☐,Pb,Ce,Na)(U⁴⁺,Mn,U⁶⁺)Fe³⁺₂(Ti,Fe³⁺)₁₈O₃₈, is a new member of the crichtonite group from the Maoniuping REE deposit, Mianning county, Sichuan province, China. It was found in fractures of lamprophyre veins and in the contact between lamprophyre and a later quartz–alkali feldspar syenite dyke with REE mineralization, and is named after its type locality. Associated minerals are microcline, albite, quartz, iron-rich phlogopite, augite, muscovite, calcite, baryte, fluorite, epidote, pyrite, magnetite, hematite, galena, hydroxylapatite, titanite, ilmenite, rutile, garnet-group minerals, zircon, allanite-(Ce), monazite-(Ce), bastnäsite-(Ce), parisite-(Ce), maoniupingite-(Ce), thorite, pyrochlore-group minerals and chlorite. Mianningite occurs as opaque subhedral to euhedral tabular crystals, up to 1–2 mm in size, black in color and streak, and with a submetallic luster. Mianningite is brittle, with a conchoidal fracture. Its average micro-indentation hardness is 83.8 kg/mm² (load 0.2 kg), which is equivalent to ~6 on the Mohs hardness scale. Its measured and calculated densities are 4.62 (8) g/cm³ and 4.77 g/cm³, respectively. Under reflected light, mianningite is grayish white, with no internal reflections. It appears isotropic and exhibits neither bireflectance nor pleochroism. The empirical formula, calculated on the basis of 38 O atoms per formula unit (apfu), is [☐_0.322(Pb_0.215Ba_0.037Sr_0.036Ca_0.010)_Σ0.298(Ce_0.128La_0.077Nd_0.012)_Σ0.217 (Na_0.127K_0.036)_Σ0.163]_Σ1.000(U⁴⁺_0.447Mn_0.293U⁶⁺_0.112Y_0.091Zr_0.023Th_0.011)_Σ0.977(Fe³⁺_1.224Fe²⁺_0.243Mg_0.023P_0.008Si_0.006☐_0.496)_Σ2.000(Ti_12.464Fe³⁺_5.292V⁵⁺_0.118Nb_0.083Al_0.026Cr³⁺_0.017)_Σ18.000O₃₈. Mianningite is trigonal, belongs to the space group $R3¯$⁠, and has unit-cell parameters a = 10.3462(5) Å, c = 20.837(2) Å, V = 1931.65 (20) ų, and Z = 3. The structure was solved (R₁ = 0.070) using reflections with I > 2σ(I) on a heated crystal; it is isostructural with the other members of the crichtonite group. Mianningite can be considered as the analogue of mapiquiroite with the M0 site preferentially vacant. Its eight strongest X-ray powder-diffraction lines [d in Å(I/I₀)(hkl)] are 2.627(100) (125), 2.144 (100) (135), 3.065(75) (025), 2.254 (70) (028), 1.545(60) (336), 2.883(55) (116), 2.476 (55) (027), and 1.705 (55) (146).