Molecular structure of the phosphate mineral koninckite - a vibrational spectroscopic study
2018
We have undertaken a study of the mineral koninckite from
Litosice (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) center dot 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 nu 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 nu(2)(delta) 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 nu(3) 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 nu(1) 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 nu(4) (delta) PO(4)(3-)triply
degenerate out-of-plane bending vibrations; weak band at 570
cm(-1) may coincide with the delta Fe-O bending vibration.
Raman bands and shoulders at 453, 443, 419, and 400 cm(-1) are
assigned to the nu(2) (delta) PO43- doubly degenerate in-plane
bending vibrations. Raman bands at 385, 346, 324, 309, 275,
252, and 227 cm(-1) are assigned to the nu Fe-O stretching
vibrations in FeO6 octahedra. Raman bands at 188, 158, 140,
112, 89, and 73 cm(-1) are assigned to lattice vibrations.
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