Several compounds of the Na₂(Mn_1−xFe²⁺_x)₂Fe³⁺(PO₄)₃ solid solution have been hydrothermally synthesized at 400 °C and 1 kbar; pure alluaudite-like compounds have been obtained for x = 0.00, 0.25, 0.50, 0.75, and 1.00. Rietveld refinements of the powder X-ray diffraction patterns indicate the presence of Na⁺ at the A1 and A2′ sites, Mn²⁺ and Fe²⁺ at the M1 site, and Mn²⁺, Fe²⁺, and Fe³⁺ at the M2 site. The presence of small amounts of Na⁺ at the M1 site and Mn²⁺ at the A1 site indicates a partially disordered distribution of these cations. An excellent linear correlation has been established between the M1-M2 distance and the energy of the infrared band attributed to the M²⁺-O vibrations. The Mössbauer spectra, measured between 85 and 295 K, were analyzed in terms of a model which includes the next-nearest neighbor interactions at the M2 and M1 crystallographic sites. Fe²⁺ and Fe³⁺ isomer shifts are typical of the alluaudite structure and exhibit the expected second-order Doppler shift. The derived iron vibrating masses and Mössbauer lattice temperatures are within the range of values expected for iron cations in an octahedral environment. The Fe²⁺ and Fe³⁺ quadrupole splittings are typical of the alluaudite structure, and the temperature dependence of the Fe²⁺ quadrupole splitting was fit with the Ingalls model, which yielded a ground state orbital splitting of ca. 460 to 735 cm⁻¹ for the Fe²⁺ sites. The isomer shifts and quadrupole splittings of Fe²⁺ at the M1 site are larger than those of Fe²⁺ at M2, indicating that the M1 site is both larger and more distorted than the M2 site.

This paper reports the crystal structure of tetra-sodium diiron tris(phosphate), Na(4)Fe(2+)Fe(3+)(PO(4))(3), which has been synthesized hydro-thermally at 773 K and 0.1 GPa. The crystal structure has been refined in the space group Rc and is identical to that of γ-NASICON. The heteropolyhedral framework is based on a regular alternation, in three dimensions, of corner-sharing PO(4) tetra-hedra and FeO(6) octa-hedra, constituting so-called 'lantern units' stacked along the c axis. The Na(+) cations are distributed over two crystallographic sites: the six-coordinated Na1 site which lies between two 'lantern units', and the eight-coordinated Na2 site which lies at the same z value as the P site.

Several compounds of the NaMn(Fe_1–xIn_x)₂(PO₄)₃ solid solution were synthesized by solid state reaction in air; pure alluaudite-like compounds were obtained for x = 0.00 to 1.00. X-ray Rietveld refinements indicate the presence of Na⁺ at the A1 and A2′ sites, Mn²⁺ at the M1 site, and Fe²⁺, Fe³⁺, and In³⁺ at the M2 site. The presence of small amounts of In³⁺ at the M1 site, and Mn²⁺ at the M2 site, indicates a partially disordered distribution between these cations. A good correlation was also established between the M1–M2 bond distance and the β angle of the alluaudite-like compounds. The disordered distribution of Fe²⁺, Fe³⁺, and In³⁺ at the M2 site is confirmed by the broadness of the infrared absorption bands. The Mössbauer spectra, measured between 90 and 295 K, were analyzed in terms of a model that takes into account the next-nearest neighbor interactions around the M2 crystallographic site. In all cases these spectra reveal the unexpected presence of small amounts of Fe²⁺ at the M2 site, an amount that decreases as the In³⁺ content increases. The Fe²⁺ and Fe³⁺ isomer shifts are typical of the alluaudite structure and vary with temperature, as expected from a second-order Doppler shift. The derived iron vibrating masses and Mössbauer lattice temperatures are within the expected range of values for iron cations in an octahedral environment. The Fe²⁺ and Fe³⁺ quadrupole splittings are also typical of the alluaudite structure and the temperature dependence of the Fe²⁺ quadrupole splitting was fit with the model of Ingalls (1964), which yielded a ground state orbital splitting of ca. 380 to 570 cm⁻¹ for the Fe²⁺ sites.

New IMA CNMNC guidelines were established for minerals crystallizing on burning coal dumps. These phases are now acceptable as minerals, if it can be proven that the fire responsible for the formation of these phases is the result of natural events. In that case, these substances have to be treated as normal new mineral species, and their complete characterization has to be submitted to the CNMNC via the new mineral proposal form. The authors are specifically asked, however, to give strong arguments in the proposal clearly demonstrating the non-anthropogenic origin of the burning process.