High-temperature Fe oxidation coupled with redistribution of framework cations in lobanovite, KNa(FeMgNa)Ti(SiO)O(OH) - the first titanosilicate case.

The high-temperature (HT) behaviour of lobanovite, KNa(FeMgNa)Ti(SiO)O(OH), 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.