High-temperature behaviour of fedorite, Na_2.5(Ca_4.5Na_2.5)[Si₁₆O₃₈]F₂⋅2.8H₂O, from the Murun Alkaline Complex, Russia

Abstract The thermal behaviour of fedorite from the Murun massif, Russia, has been investigated by means of electron probe microanalysis (EPMA), differential thermal analysis (DTA), thermogravimetry (TG), in situ high-temperature single-crystal X-ray diffraction (HT-SCXRD), ex situ high-temperature Fourier-transform infrared spectroscopy (HT-FTIR). The empirical chemical formula of the sample of fedorite studied is: (Na 1.56 K 0.72 Sr 0.12 ) Σ2.40 (Ca 4.42 Na 2.54 Mn 0.02 Fe 0.01 Mg 0.01 ) Σ7.00 (Si 15.98 Al 0.02 ) Σ16.00 (F 1.92 Cl 0.09 ) Σ2.01 (O 37.93 OH 0.07 ) Σ38.00 ⋅2.8H 2 O. The TG curve provides a total mass decrease of ~5.5%, associated with dehydration and defluorination processes from 25 to 1050°C. Fedorite crystallises in space group P $\bar{1}$ and has: a = 9.6458(2), b = 9.6521(2), c = 12.6202(4) Å, α = 102.458(2), β = 96.2250(10), γ = 119.9020(10)° and cell volume, V = 961.69(5) Å 3 . The HT-SCXRD was carried out in air in the 25–600°C range. Overall, a continuous expansion of the unit-cell volume was observed although the c cell dimension slightly decreases in the explored temperature range. Structure refinements indicated that the mineral undergoes a dehydration process with the loss of most of the interlayer H 2 O from 25 to 300°C. The HT-FTIR spectra confirmed that fedorite progressively dehydrates until 700°C.