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Betalomonosovite: chemical and structural variability and genesis

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The crystal structures of four betalomonosovite samples from peralkaline pegmatites of the Khibiny alkaline complex, Kola Peninsula, Russia, were studied using single-crystal X-ray diffraction. The samples represent different chemical and structural varieties of betalomonosovite. The suggested general formula of betalomonosovite, based on the empirical data known to date, is Na5 + x Ti4 [Si2 O7]2 [PO3 (OH)]2 - y [PO2 (OH)2 ]y O2 [(OH, F)2 - z Oz], where 0≤ x ≤ 2, 0 ≤ y ≤ 1 and 0 ≤ z ≤ 1. Betalomonosovite is a transformation product of lomonosovite, Na10 Ti4 (Si2 O7)2 (PO4)2 O4, due to the leaching of Na and protonation of O atoms of phosphate groups and Ti–O–Ti bridges. Distinctive features of betalomonosovite are: (a) the presence of Hþ in the form of speciesdefining acid phosphate anion(s) [PO2 (OH)2] ± [PO3 (OH)]; (b) the presence of OH- anion at the Ti–O–Ti bridges in the O sheet of the HOH block and c) deficiency of Na (as compared to lomonosovite) in an amount related to the degree of protonation. A hypothetical end-member product of lomonosovite alteration with the simplified formula Na4 Ti4 [Si2 O7]2 [PO2 (OH)2]2 O2 (OH)2 has not been reported in Nature yet. The ways and the degree of alteration may vary significantly, resulting in significant variations in both crystal structure and chemical composition, including the Na content, the degree of the substitution of O by OH, unit-cell dimensions and splitting of the P, O and/or Na sites.
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Keywords: BETALOMONOSOVITE; CRYSTAL STRUCTURE; HETEROPHYLLOSILICATE; KHIBINY ALKALINE COMPLEX; KOLA PENINSULA; LOMONOSOVITE; PERALKALINE ROCK; TITANOSILICATE; TRANSFORMATION MINERAL

Document Type: Research Article

Publication date: 01 April 2018

This article was made available online on 22 June 2018 as a Fast Track article with title: "Betalomonosovite: chemical and structural variability and genesis".

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  • The European Journal of Mineralogy publishes original papers, review articles and letters dealing with the mineralogical sciences s.l. These include primarily mineralogy, petrology, geochemistry, crystallography and ore deposits, as well as environmental, applied and technical mineralogy. Nevertheless, papers in any related field, including cultural heritage, will be considered.
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