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 (original) (raw)

Original paper

Holtstam, Dan; Bindi, Luca; Hålenius, Ulf; Andersson, Ulf B.

Abstract

Delhuyarite-(Ce) is a new mineral (IMA no. 2016-091) with ideal formula Ce4 Mg(Fe3+ 2W)[](Si2 O7)2 O6 (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%) La2 O3 14.58, Ce2 O3 23.29, Pr2 O3 1.89, Nd2 O3 6.13, Sm2 O3 0.74, Gd2 O3 0.37, Dy2 O3 0.03, Er2 O3 0.04, Yb2 O3 0.12, Y2 O3 0.22, CaO 0.76, Fe2 O3 12.86, MgO 2.43, Al2 O3 0.73, SiO2 18.16, TiO2 0.09, WO3 15.53, H2 Ocalc 1.33, F 0.05, Cl 0.03, O=(F, Cl) 0.03, sum 99.35, corresponding to an empirical formula: (Ce1.89 La1.19 Nd0.48 Pr0.15 Sm0.06 Gd0.03 Y0.03 Ca0.18)Σ4.01 (Fe3+ 2.14W0.89Mg0.80Al0.19Ti0.02)Σ4.04 Si4.01 O20 (OH1.96 F0.04)Σ2, based on 22 O atoms per formula unit (apfu). The presence of H2 O is confirmed by IR-spectroscopy, from a strong absorption band at 3495 cm1. 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) Å3 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 chevkinitesubgroup 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 M 1 site as in polyakovite-(Ce); the composition of the M 2, M 3 and M 4 sites is [(Fe3+ 2W[] ], with M2 being 50% vacant.

Keywords

crystal structuregroupdelhuyarite-(ce)bastnästungstenree silicateswedenchevkiniteelectron microprobe analysisnew mineral species