Heat-Induced Mn2+ and Fe2+ Oxidation in Heterophyllosilicates with Porous Structure: Kupletskite and Kupletskite-(Cs)

The crystal-chemical behavior of two layered titanosilicate minerals with porous crystal structures: kupletskite, K2NaMn72+Ti2(Si4O12)2O2(OH)4F, and kupletskite-(Cs), Cs2NaMn72+Ti2(Si4O12)2O2(OH)4F, was investigated under high-temperature conditions using single-crystal and powder X-ray diffraction; infrared, optical absorption and Mössbauer spectroscopy and electron-microprobe analysis. Both minerals undergo topotactic transformation to dehydroxylated and oxidized high-temperature (HT) modifications at temperature above 500 °C while maintaining the basic bond topology of the astrophyllite structure-type. The high-temperature structures show contraction of the unit-cell parameters similar to that of Fe2+-dominant astrophyllite, indicating that Mn2+ oxidizes along with Fe2+. The oxidation of Mn2+ is confirmed by the increase of the Mn3+-related absorption (in optical spectra) that is inversely correlated with the intensity of O‒H bands in the infrared spectra. The Fe,Mn-oxidation is also evident by the contraction of the M(2), M(3) and M(4)O6 octahedra. The M(1)‒O bond length increases slightly, indicating a preference for mono- and divalent cations to occupy the M(1) site in the heated structure; this may be due to site-selective oxidation and/or migration of unoxidized cations (as previously shown for lobanovite) to this site. The role of extra framework A-site cations (K, Cs) in thermal expansion of these minerals is discussed.