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Mineralogical ClassificationCrystal structure of the OH-dominant gadolinite-(Y) analogue

3rd Oct 2017 14:21 UTCMarco E. Ciriotti Manager

Reference:

▪ Chukanov, N.V., Aksenov, S.M., Rastvetaeva, R.M., Kristiansen, R., Pekov, I.V, Belakovskiy, D.I., Van, V.K., Bychkova, Y.V., Bychkova, Y.V., Britvin, S.N. (2017): Crystal structure of the OH-dominant gadolinite-(Y) analogue, (Y,Ca)2(Fe,☐)Be2Si2O8(OH,O)2 from Heftetjern pegmatite, Norway. Acta Crystallographica, B73, 899-906.

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 REE-minerals. The empirical formula based on 10 O apfu is (Y1.285Ca0.55Ce0.07La0.04Nd0.01)tot1.955Fe2+0.57Be2.02Si1.995O8.48(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. Density calculated using the empirical formula is 3.903 g/cm3. The crystal structure was refined to R = 0.0217 for 776 > 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. Refined crystal-chemical formula is (Z = 2): A(Y1.25Ca0.55Ce0.2)X(Fe2+0.57☐0.43)ZBe2TSi2O8Ø[(OH)0.86O0.59(OH)*0.55]. Possible orientation and local environment of hydroxyl group were suggested based on bond-valence sum calculations and geometrical analysis of the crystal structure. The IR spectrum confirms disordering of hydrogen atoms. OH-Gad is seems to be a potentially new mineral – the first simultaneously hydroxyl- and iron-dominant member of gadolinite subgroup. It is a OH-analogue of gadolinite-(Y) and Fe2+-analogue of hingganite-(Y).