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Acta Crystallogr B Struct Sci Cryst Eng Mater. 2017 Oct 1;73(Pt 5):899-906. doi: 10.1107/S2052520617006588. Epub 2017 Sep 16.

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

Author information

1
Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow Region 142432, Russian Federation.
2
Faculty of Geology, St Petersburg State University, University Embankment 7/9, St Petersburg 199034, Russian Federation.
3
Institute of Crystallography of Federal Scientific Research Centre `Crystallography and Photonics', Russian Academy of Sciences, Leninskii Prospekt 59, Moscow 119333, Russian Federation.
4
PO Box 32, N-1650 Sellebakk, Norway.
5
Faculty of Geology, Moscow State University, Vorobievy Gory, Moscow 119991, Russian Federation.
6
Fersman Mineralogical Museum of the Russian Academy of Sciences, Leninsky Prospekt 8-2, Moscow 117071, Russian Federation.
7
Institute of Experimental Mineralogy, Russian Academy of Sciences, Chernogolovka, Moscow Region 142432, Russian Federation.
8
Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences, Staromonetny Per. 35, Moscow 119017, Russian Federation.

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+0.570.43)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+0.570.43)ZBe2TSi2O8Ø[(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).

KEYWORDS:

Heftetjern; OH-dominant gadolinite-(Y); gadolinite-datolite group

PMID:
28980995
DOI:
10.1107/S2052520617006588

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