Kryzaite, Na4(MgCr)(PO4)3 – a new mineral from the Morasko IAB-MG iron meteorite (Poland) and the first natural NASICON compound

Abstract Kryzaite, a new mineral with the ideal formula Na4(MgCr)(PO4)3 and the empirical formula Na3.93(Mg0.94Fe2+0.07Mn2+0.03 Cr3+1.02)Σ2.06P2.99O12 is the third new phosphate to be discovered in the graphite-troilite nodules of the iron meteorite Morasko, which belongs to the IAB-MG meteorite group. The other two minerals are moraskoite, Na2Mg(PO4)F, and czochralskiite, Na4Ca3Mg(PO4)4. The kryzaite structure was refined based on a single-crystal X-ray diffraction study of a 0.02 × 0.012 × 0.01 mm grain. It is a trigonal, space group R-3c, a = 8.8509(6)Å, c = 21.2564(18)Å, V = 1442.1(2) Å3, Z = 6. A synthetic analogue, belonging to the NASICON (Na Super Ionic Conductor) compound type, is known and has a potential application in the production of Na-ion batteries. The kryzaite structural framework is formed by [(Mg/Cr)O6] octahedra and (PO4) tetrahedra jointed at the vertices and contains channels with Na-occupied sites. The high rate of low-temperature Na diffusion determines the high ionic conductivity and high chemical reactivity of kryzaite type compounds. Kryzaite preservation is defined by the rapid formation of a stable rim that preserves and protects the mineral from later alteration. Kryzaite forms transparent green grains with a vitreous lustre measuring up to 30 μm in size in the central part of aggregates. The rim of these aggregates is formed by a transparent brown phase with a vitreous lustre and the empirical formula Na1.50(Mg0.98Mn2+0.07)(Fe3+0.77Cr3+1.01)(PO4)3. The calculated density of kryzaite is 3.14 g·cm-3, based on structural data and the empirical formula. The following bands are observed in the Raman spectrum of kryzaite (cm-1): 1116, 1111, 1066 ν3(PO4)3-; 1022 ν3(PO4)3-/ν1(PO4)3-; 979 ν1(PO4)3; 597 ν4(PO4)3; 459, 437 ν2(PO4)3-; 341 ν(CrO6)9-; 266, 231, 163 νT(PO4)3-, νR(PO4)3-, νNa-O. Kryzaite in the Morasko meteorite most likely crystallized from a phosphate melt at a temperature of about 950-1000°C, as well as the primary Na-containing phosphates - brianite, moraskoite, czochralskiite and merrillite.