High-temperature Fe oxidation coupled with redistribution of framework cations in lobanovite, K₂Na(Fe²⁺₄Mg₂Na)Ti₂(Si₄O₁₂)₂O₂(OH)₄– the first titanosilicate case

Abstract

OH) 4 , was studied using in situ powder X-ray diffraction in the temperature range 25-1000 C and ex situ single-crystal X-ray diffraction of 17 crystals quenched from different temperatures. HT iron oxidation associated with dehydroxylation starts at 450 C, similar to other ferrous-hydroxy-rich heterophyllosilicates such as astrophyllite and bafertisite. A prominent feature of lobanovite HT crystal chemistry is the redistribution of Fe and Mg+Mn cations over the M(2), M(3), M(4) sites of the octahedral (O) layer that accompanies iron oxidation and dehydroxylation. This HT redistribution of cations has not been observed in titanosilicates until now, and seems to be triggered by the need to maintain bond strengths at the apical oxygen atom of the TiO 5 pyramid in the heteropolyhedral (H) layer during oxidationdehydroxylation. Comparison of the HT behaviour of lobanovite with fivecoordinated Ti and astrophyllite with six-coordinated Ti shows that the geometry of the Ti polyhedron plays a key role in the HT behaviour of heterophyllosilicates. The thermal expansion, geometrical changes and redistribution of site occupancies which occur in lobanovite under increasing temperature are reported. A brief discussion is given of minerals in which the cation ordering (usually for Fe and Mg) occurs together with iron oxidationdehydroxylation at elevated temperatures: micas, amphiboles and tourmalines. Now this list is expanded by the inclusion of titanosilicate minerals.

Extraordinary structural complexity of ilmajokite: a multilevel hierarchical framework structure of natural origin

“…Due to their microporosity and catalytic activity, titanosilicates continue to attract considerable attention in the fields of materials science and nanochemistry (Rocha & Anderson, 2000;Noh et al, 2012;Milyutin et al, 2017;Oleksiienko et al, 2017;Přech, 2018;Cuko et al, 2018;Figueiredo et al, 2018). At the same time, they are of great interest from the viewpoint of mineralogy and geochemistry given their large diversity mostly in alkaline rocks such as those occurring in the alkaline massifs of the Kola peninsula, Russia, and Mont-Saint-Hilaire, Quebec, Canada (Cá mara et al, 2017;Selivanova et al, 2018;Zolotarev et al, 2018;Andrade et al, 2018;Lykova et al, 2018;Pakhomovsky et al, 2018;Pekov et al, 2019;Zhitova et al, 2019). Indeed, most titanosilicate materials used in industry were found as minerals before their unique properties were recognized.…”