Delhuyarite-(Ce)–Ce4 Mg(Fe3+ 2W) [] (Si2 O7)2O6 (OH)2 – a new mineral of the chevkinite group, from the Nya Bastnäs Fe–Cu–REE deposit, Sweden

Abstract

Delhuyarite-(Ce) is a new mineral (IMA no. 2016-091) with ideal formula Ce 4 Mg(Fe 3+ 2 W)□(Si 2 O 7 ) 2 O 6 (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 Bastnas Fe–Cu–REE deposit (Vastmanland, Sweden), include cerite-(Ce), tremolite–actinolite, percleveite-(Ce), bastnasite-(Ce), ferriallanite-(Ce), tornebohmite-(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%) La 2 O 3 14.58, Ce 2 O 3 23.29, Pr 2 O 3 1.89, Nd 2 O 3 6.13, Sm 2 O 3 0.74, Gd 2 O 3 0.37, Dy 2 O 3 0.03, Er 2 O 3 0.04, Yb 2 O 3 0.12, Y 2 O 3 0.22, CaO 0.76, Fe 2 O 3 12.86, MgO 2.43, Al 2 O 3 0.73, SiO 2 18.16, TiO 2 0.09, WO 3 15.53, H 2 O calc 1.33, F 0.05, Cl 0.03, O=(F, Cl) −0.03, sum 99.35, corresponding to an empirical formula: (Ce 1.89 La 1.19 Nd 0.48 Pr 0.15 Sm 0.06 Gd 0.03 Y 0.03 Ca 0.18 ) Σ4.01 (Fe 3+ 2.14 W 0.89 Mg 0.80 Al 0.19 Ti 0.02 ) Σ4.04 Si 4.01 O 20 (OH 1.96 F 0.04 ) Σ2 , based on 22 O atoms per formula unit (apfu). The presence of H 2 O is confirmed by IR-spectroscopy, from a strong absorption band at 3495 cm −1 . Delhuyarite-(Ce) is monoclinic, space group C 2 /m , with unit-cell parameters a =13.6020(6) A, b = 5.7445(3) A, c = 10.9996(5) A, β = 100.721(4)°, V = 844.47 (6) A 3 and Z = 2 (data for natural crystal). The crystal structure was refined to an R 1 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 W 6+ = 0.89 apfu. In delhuyarite-(Ce), Mg is dominant at the M 1 site as in polyakovite-(Ce); the composition of the M 2, M 3 and M 4 sites is [(Fe 3+ 2 W□], with M 2 being 50% vacant.