Deprotonation and order-disorder reactions as a function of temperature in a phengite 3T (Cima Pal, western Alps) by neutron diffration and Mössbauer spectroscopy

The results on deprotonation and cation partitioning as a function of temperature in the tetrahedral and octahedral sites of a phengite 3T, (K_0.94Na_0.02) _Σ0.96 (Al_1.43Mg_0.33Fe_0.25 Σ_2.01(Si_3.47Al_0.53) Σ₄O₁₀(OH)₂, from Cima Pal (western Alps) are presented and discussed. They have been obtained by a synergic approach based on neutron powder diffraction from 293 K to 873 K, and on Mössbauer spectroscopy at ambient conditions, of the sample before and after heating. Order is observed at room temperature in both tetrahedral (Si fully occupies T1) and octahedral (Al almost fills M2) sites. Upon heating, deprotonation [mainly according to the reaction 2[(OH)- + Fe²⁺] → 2O^2- + 2Fe³⁺ + H₂↑], Fe oxidation and inter-site cation re-ordering reactions take place. Al moves by ∼ 0.1 atoms from M2 to M3, and is replaced by Mg and Fe; Fe²⁺ and Fe³⁺ occupy different M-sites, after cooling, whereas Fe²⁺ was partitioned on two sites, one site shared with Fe³⁺, before heating.