Crocobelonite, CaFe₂³⁺(PO₄)₂O, a new oxyphosphate mineral, the product of pyrolytic oxidation of natural phosphides

Crocobelonite, CaFe23+(PO4)(2)O, is a new natural oxyphosphate discovered in the pyrometamorphic complexes of the Hatrurim Formation in Israel and Jordan. Crocobelonite-bearing assemblages contain a series of anhydrous Fe-Ni phosphates, hematite, diopside, anorthite, and phosphides-barringerite Fe2P, transjordanite Ni2P, murashkoite FeP, halamishite Ni5P4, and negevite NiP2. Crocobelonite forms submillimeter-sized aggregates of prismatic to acicular crystals of saffron-red to pinkish-red color. There are two polymorphic modifications of the mineral whose structures are interrelated by the unit-cell twinning. Crocobelonite-2O is orthorhombic, Pnma, a = 14.2757(1), b = 6.3832(1), c = 7.3169(1) angstrom, V 666.76(1) angstrom(3), Z = 4. This polymorphic modification is isotypic with synthetic oxy-phosphates A V-2(3+)(PO4)(2)O where A = Ca, Sr, Cd. The crystal structure has been refined to R-B = 0.71% based on powder XRD data, using the Rietveld method and the input structural model obtained from the single-crystal study. Chemical composition (electron microprobe, wt%) is: CaO 16.03, MgO 0.56, Fe2O3 43.37, Al2O3 0.33, SiO2 0.32, P2O5 39.45, Total 100.06. The empirical formula based on O = 9 apfu is Ca-1.02(Fe1.943+Mg0.05Al0.02)(2.01)(P1.98Si0.02)(2.00)O-9.00 with D-calc = 3.555 g/cm(3). The strongest lines of powder XRD pattern [d(angstrom)(I)(hkl)] are: 6.54(16)(200), 5.12(26)(201), 3.549(100)(102), 3.200(50) (401), 2.912(19)(220), 2.869(40)(411), 2.662(21)(501). Crocobelonite-1M is monoclinic, P2(1)/m, a = 7.2447(2), b = 6.3832(1), c = 7.3993(2) angstrom, beta = 106.401(2)degrees, V = 328.252(14) angstrom(3), Z = 2. This polymorphic modification does not have direct structural analogs. Its crystal structure has been solved and refined based on the single-crystal data to R-1 = 1.81%. Chemical composition is: CaO 15.56, MgO 0.16, NiO 0.78, Fe2O3 41.28, Al2O3 0.45, V2O3 0.42, Cr2O3 0.23, TiO2 0.79, P2O5 39.94, Total 99.61, corresponding to the empirical formula (O = 9 apfu) Ca-0.99(Fe1.853+Ni0.04Ti0.04Al0.03V0.023+Cr0.01Mg0.01)(2.00)P2.01O9.00 with D-calc = 3.604 g/cm(3). The strongest lines of powder XRD pattern [d(angstrom)(I)(hkl)] are 6.98(17)(100), 4.40(22)(101), 3.547(100)((2) over bar 01), 3.485(21)(200), 3.195(50)(020), 2.855(38)(102), 2.389(33)((1) over bar 22). Crocobelonite represents a novel type of phosphate mineral formed by oxidation of phosphide minerals at temperatures higher than 1000 degrees C and near-atmospheric pressure (pyrolytic oxidation).