Delhuyarite-(Ce) – Ce₄Mg(Fe³⁺₂W)□(Si₂O₇)₂O₆(OH)₂ – a new mineral of the chevkinite group, from the Nya Bastnäs Fe–Cu–REE deposit, Sweden

Delhuyarite-(Ce) is a new mineral (IMA no. 2016-091) with ideal formula Ce₄Mg(Fe³⁺₂W)□(Si₂O₇)₂O₆(OH)₂. It is named after Juan and Fausto de Elhuyar (Delhuyar), chemists and metallurgists, who in 1783 isolated tungsten metal for the first time. Associated minerals in the only known sample, from the Nya Bastnäs Fe–Cu–REE deposit (Västmanland, Sweden), include cerite-(Ce), tremolite‒actinolite, percleveite-(Ce), bastnäsite-(Ce), ferriallanite-(Ce), törnebohmite-(Ce), magnetite, chalcopyrite, quartz and scheelite. Delhuyarite-(Ce), which forms subhedral crystals up to 0.3 mm long, is brown–black with a dark brown streak and translucent with an adamantine lustre. It is pleochroic in black to rust red and optically biaxial (−). Calculated density and mean refractive index are 5.20 g∙cm⁻³ and 1.94, respectively. Chemical analyses (electron microprobe) gave (in wt%) La₂O₃ 14.58, Ce₂O₃ 23.29, Pr₂O₃1.89, Nd₂O₃ 6.13, Sm₂O₃ 0.74, Gd₂O₃ 0.37, Dy₂O₃ 0.03, Er₂O₃ 0.04, Yb₂O₃ 0.12, Y₂O₃ 0.22, CaO 0.76, Fe₂O₃ 12.86, MgO 2.43, Al₂O₃ 0.73, SiO₂ 18.16, TiO₂ 0.09, WO₃ 15.53, H₂O_calc 1.33, F 0.05, Cl 0.03, O=(F, Cl) −0.03, sum 99.35, corresponding to an empirical formula: (Ce_1.89La_1.19Nd_0.48Pr_0.15Sm_0.06Gd_0.03Y_0.03Ca_0.18)_Σ4.01(Fe³⁺_2.14W_0.89Mg_0.80Al_0.19Ti_0.02)_Σ4.04Si_4.01O₂₀ (OH_1.96F_0.04)_Σ2, based on 22 O atoms per formula unit (apfu). The presence of H₂O is confirmed by IR-spectroscopy, from a strong absorption band at 3495 cm⁻¹. Delhuyarite-(Ce) is monoclinic, space group C2/m, with unit-cell parameters a =13.6020(6) Å, b = 5.7445(3) Å, c = 10.9996(5) Å, β = 100.721(4)°, V = 844.47 (6) ų and Z = 2 (data for natural crystal). The crystal structure was refined to an R₁ index of 3.9% (natural crystal) and 1.8% (annealed). Delhuyarite-(Ce) has the same structural topology as chevkinite-subgroup minerals, e.g. chevkinite-(Ce). It is the only mineral of the group with a significant content of W⁶⁺ = 0.89 apfu. In delhuyarite-(Ce), Mg is dominant at the M1 site as in polyakovite-(Ce); the composition of the M2, M3 and M4 sites is [(Fe³⁺₂W□], with M2 being 50% vacant.