Crystal structure of the OH-dominant gadolinite-(Y) analogue (Y,Ca)2(Fe,□)Be2Si2O8(OH,O)2 from Heftetjern pegmatite, Norway

Abstract

A hydroxyl-dominant analogue of gadolinite-(Y) (OH-Gad) has been discovered in the Heftetjern granitic pegmatite, southern Norway, in association with late-stage rare-earth-element containing minerals. The empirical formula, based on ten O atoms per formula unit, is (Y1.285Ca0.55Ce0.07La0.04Nd0.01)Σ1.955(Fe2+ 0.57□0.43)Be2.02Si1.995O8.48(OH)1.52. The mineral is monoclinic, space group P21/c, a = 4.7514 (10), b = 7.5719 (16), c = 9.9414 (2) Å, β = 90.015 (4)°, V = 357.663 (3) Å3 and Z = 2. The density calculated using the empirical formula is 3.903 g cm−3. The crystal structure was refined to R = 0.0217 for 776 reflections with I > 2σ(I). OH-Gad is isostructural with gadolinite-(Y) and it is characterized by the predominance of OH− over O2− at the anionic Ø-site. The refined crystal-chemical formula is: A (Y1.25Ca0.55Ce0.2) X (Fe2+ 0.57□0.43) Z Be2 T Si2O8 Ø[(OH)0.86O0.59(OH)*0.55] (Z = 2). The possible orientation and local environment of the hydroxyl group were suggested based on bond-valence sum calculations and geometrical analysis of the crystal structure. The infrared spectrum confirms disordering of H atoms. OH-Gad seems to be a potentially new mineral, the first simultaneously hydroxyl- and iron-dominant member of the gadolinite subgroup. It is an OH-analogue of gadolinite-(Y) and an Fe2+-analogue of hingganite-(Y).