Molecular structure of the phosphate mineral koninckite - a vibrational spectroscopic study

J Jirásek,D Matýsek,R.L Frost,J Čejka,X Ruan,L Vrtiška

doi:10.3190/jgeosci.243

Abstract

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.

Highlights

Koninckite is a natural phosphate mineral of formula Fe3+(PO4)∙2.75H2O
Raman spectroscopy shows the presence of PO43– -units as identified by the position of the Raman peaks
The observation of multiple phosphate bands supports the concept of nonequivalent phosphate units in the koninckite structure

Summary

Introduction

Koninckite is a natural phosphate mineral of formula Fe3+(PO4)∙2.75H2O. It was named by Cesàro (1884) after Professor Laurent Guillaume de Koninck (1809–1887), notable Belgian paleontologist and chemist. The mineral forms yellowish, white and colorless coatings, veins and rarely crystal aggregates. Koninckite belongs to the tetragonal crystal system. The crystal structure of koninckite was solved recently by Plášil et al (2015) from synchrotron powder X-ray diffraction data. Investigated material from Kociha (Slovakia) is tetragonal, with the space group P41212. Its crystal structure is built as a heteropolyhedral framework with zeolite-like tunnels along [001]. The Fe3+ ions in the structure are in octahedral coordination by H2O molecules and oxygen ions of the PO43– tetrahedra. Koninckite should be an antiferromagnetic semiconductor, at least at low temperatures

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