Molecular structure of the phosphate mineral koninckite – A vibrational spectroscopic study

Abstract

We have undertaken a study of the mineral koninckite from Litošice (Czech Republic), a hydrated ferric phosphate, using a combination of scanning electron microscopy with electron probe micro-analyzer (wavelength-dispersive spectroscopy) and vibrational spectroscopy. Chemical analysis shows that studied koninckite is a pure phase with an empirical formula Fe3+0.99(PO4)1.00∙2.75 H2O, with minor enrichment in Al, Ca, Ti, Si, Zn, and K (averages 0.00X apfu). Raman bands and shoulders at 3495, 3312, 3120, and 2966 cm–1 and infrared bands and shoulders at 3729, 3493, 3356, 3250, 3088, 2907, and 2706 cm–1 are assigned to the ν OH stretching of structurally distinct differently hydrogen bonded water molecules, A Raman band at 1602 cm–1 and shoulders at 1679, 1659, 1634, and 1617 cm–1 and infrared bands at 1650 and 1598 cm–1 are assigned to the ν2 (δ) H2O bending vibrations of structurally distinct differently hydrogen bonded water molecules. Raman shoulders at 1576, 1554, 1541, 1532, and 1520 cm–1 and infrared shoulders at 1541 and 1454 cm–1 may be probably connected with zeolitically bonded water molecules located in the channels. Raman bands and shoulders at 1148, 1132, 1108, 1063, 1048, and 1015 cm–1 and an infrared band and shoulders at 1131, 1097, 1049, and 1017 cm–1 are assigned to the ν3 PO43– triply degenerate antisymmetric stretching vibrations. A Raman band and a shoulder at 994 and 970 cm–1, respectively, and an infrared band and a shoulder at 978 and 949 cm–1, respectively, are assigned to the ν1 PO43– symmetric stretching vibrations. Infrared shoulders at 873, 833, and 748 cm–1 are assigned to libration modes of water molecules. Raman bands and shoulders at 670, 648, 631, 614, 600, 572, and 546 cm–1 and infrared bands at 592 and 534 cm–1 are assigned to the ν4 (δ) PO43– triply degenerate out-of-plane bending vibrations; weak band at 570 cm–1 may coincide with the δ Fe–O bending vibration. Raman bands and shoulders at 453, 443, 419, and 400 cm–1 are assigned to the ν2 (δ) PO43– doubly degenerate in-plane bending vibrations. Raman bands at 385, 346, 324, 309, 275, 252, and 227 cm–1 are assigned to the ν Fe–O stretching vibrations in FeO6 octahedra. Raman bands at 188, 158, 140, 112, 89, and 73 cm–1 are assigned to lattice vibrations.