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Crystal structure of the OH‐dominant gadolinite‐(Y) analogue (Y,Ca) 2 (Fe,□)Be 2 Si 2 O 8 (OH,O) 2 from Heftetjern pegmatite, Norway
Author(s) -
Chukanov Nikita V.,
Aksenov Sergey M.,
Rastsvetaeva Ramiza K.,
Kristiansen Roy,
Pekov Igor V.,
Belakovskiy Dmitriy I.,
Van Konstantin V.,
Bychkova Yana V.,
Britvin Sergey N.
Publication year - 2017
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520617006588
Subject(s) - monoclinic crystal system , isostructural , crystallography , crystal structure , chemistry , valence (chemistry) , crystal (programming language) , alkali metal , stereochemistry , organic chemistry , computer science , programming language
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 (Y 1.285 Ca 0.55 Ce 0.07 La 0.04 Nd 0.01 ) Σ1.955 (Fe 2+ 0.57 □ 0.43 )Be 2.02 Si 1.995 O 8.48 (OH) 1.52 . The mineral is monoclinic, space group P 2 1 / c , a = 4.7514 (10), b = 7.5719 (16), c = 9.9414 (2) Å, β = 90.015 (4)°, V = 357.663 (3) Å 3 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 O 2− at the anionic Ø‐site. The refined crystal‐chemical formula is: A (Y 1.25 Ca 0.55 Ce 0.2 ) X (Fe 2+ 0.57 □ 0.43 ) Z Be 2 T Si 2 O 8 Ø [(OH) 0.86 O 0.59 (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 Fe 2+ ‐analogue of hingganite‐(Y).