Manganilvaite from Dalnegorsk (Primorsky Krai, Russia): crystal structure, thermal transformation and thermodynamic data

For the first time, a comprehensive physicochemical study of manganilvaite (Pervyi Sovetskiy Mine, Dalnegorsk, Primorsky Krai) was carried out using single-crystal X-ray diffraction, electron probe and thermogravimetric analyses, IR- and Mössbauer spectroscopy. The structure of manganilvaite was refined (R = 0.0272 for 896 independent reflections with I>2σ(I)). The mineral has the composition $\left({\text{Ca}}_{0.95}{\text{Mn}}_{0.04}^{2+}{\text{Mg}}_{0.01}\right)\left({\text{Mn}}_{0.65}^{2+}{\text{Fe}}_{0.35}^{2+}\right)\left({\text{Fe}}_{1.00}^{2+}{\text{Fe}}_{1.00}^{3+}\right)\left[{\text{Si}}_{\text{2}}{\text{O}}_{\text{7}}\right]\text{O}{\left(\text{OH}\right)}_{1.00}$ . Manganilvaite is orthorhombic, space group Pnam, unit cell parameters are: a = 12.9895(7), b = 8.8337(4), c = 5.8370(3) Å, V = 669.76(6) ų. The enthalpy of formation of manganilvaite ${\text{Ca}}_{1.0}\left({\text{Mn}}_{0.7}^{2+}{\text{Fe}}_{0.3}^{2+}\right)\left({\text{Fe}}_{1.0}^{2+}{\text{Fe}}_{1.0}^{3+}\right)\left[{\text{Si}}_{\text{2}}{\text{O}}_{\text{7}}\right]\text{O}{\left(\text{OH}\right)}_{1.0}$(–3778.8 ± 7.5 kJ/mol) was determined by melt solution calorimetry; the ∆_fH⁰(298.15 K) values for ilvaite CaFe²⁺(Fe²⁺Fe³⁺)[Si₂O₇]O(OH) and manganilvaite of idealized composition CaMn²⁺(Fe²⁺Fe³⁺)[Si₂O₇]O(OH) were calculated (–3739.9 ± 8.7 and –3795.5 ± 7.2 kJ/mol, respectively), and the standard entropies and Gibbs energies of formation of manganilvaite from Dalnegorsk, ilvaite and manganilvaite of idealized composition from the elements were calculated.