Mieite–(Y), Y₄Ti(SiO₄)₂O[F,(OH)]₆, a new mineral in a pegmatite at Souri Valley, Komono, Mie Prefecture, central Japan

Abstract

Mieite–(Y), ideally Y₄Ti(SiO₄)₂O[F,(OH)]₆, was found in a pegmatite at Souri Valley, Komono, Mie Prefecture, central Japan. It occurs as an amber yellow mass with adamantine luster, approximate size of 1 cm and white streak. The mineral is associated with quartz, albite, K–feldspar, muscovite, allanite–(Ce), gadolinite–(Y), and magnesiorowlandite–(Y). Cleavage is not observed and fracture is uneven. The Mohs hardness is 6. The calculated density is 4.61 g/cm³. It is biaxial and refractive indices are α = 1.694(2) and γ = 1.715(5) with non–pleochroism. The mineral displays anomalous blue interference colors. The empirical formula is (Y_3.13Dy_0.20Gd_0.17Yb_0.08Nd_0.08Sm_0.07Er_0.07Th_0.05Tb_0.03Ho_0.03Lu_0.03Ce_0.02Tm_0.02U_0.02)_Σ4.00(Ti_0.52Al_0.44Fe_0.01)_Σ0.97(Si_1.92P_0.12)_Σ2.04O₉[F_3.83(OH)_1.91]_∑5.74 on the basis of 7 cations and 9 oxygen atoms pfu after electron microprobe (WDS), FT/IR and crystal structure analyses by means of single crystal XRD data. The raw material is significantly metamictized to give extremely weak diffraction peaks. The unit cell parameters refined from powder XRD pattern of recrystallized material are; a = 14.979(6), b = 10.548(5), c = 6.964(3) Å, V = 1100.3(8) ų and Z = 4. The 7 strongest lines in the powder XRD pattern [d(Å) (I/I₀) hkl] are; 2.68 (100) 331, 3.76 (85) 400, 3.54 (83) 002, 3.48 (82) 130, 2.16 (78) 023, 4.26 (68) 021, 5.46 (58)111. The crystal structure was refined in space group Cmcm to R₁ = 0.0825 and 0.0735 for 491 and 581 reflections with I > 2σ(I) single crystal XRD data of raw and recrystallized materials, respectively. The crystal structure of mieite–(Y) consists of infinite columns of corner–sharing TiO₆ octahedra decorated by SiO₄ tetrahedra. The columns are linked by two independent Y polyhedra with different coordination, YO₂F₅ and YO₅F₃. A coupled substitution of Ti⁴⁺ + F⁻ = Al³⁺ + □ (vacancy) was suggested for mieite–(Y). Mieite–(Y) is isostructural with the ‘yftisite’, a discredited species. Mieite–(Y) can be classified in the Dana class 52.4.4.3 and Strunz class 9.AG.25, nesosilicates with additional anions.