A vibrational spectroscopic study of the phosphate mineral lulzacite Sr2Fe2+(Fe2+,Mg)2Al4(PO4)4(OH)10.

Abstract

The mineral lulzacite from Saint-Aubin des Chateaux mine, France, with theoretical formula Sr2Fe2+(Fe2+,Mg)2Al4(PO4)4(OH)10 has been studied using a combination of electron microscopy with EDX and vibrational spectroscopic techniques. Chemical analysis shows a Sr, Fe, Al phosphate with minor amounts of Ga, Ba and Mg. Raman spectroscopy identifies an intense band at 990 cm_1 with an additional band at 1011 cm_1. These bands are attributed to the PO3_ 4 m1 symmetric stretching mode. The m3 antisymmetric stretching modes are observed by a large number of Raman bands. The Raman bands at 1034, 1051, 1058, 1069 and 1084 together with the Raman bands at 1098, 1116, 1133, 1155 and 1174 cm_1 are assigned to the m3 antisymmetric stretching vibrations of PO3_ 4 and the HOPO2_ 3 units. The observation of these multiple Raman bands in the symmetric and antisymmetric stretching region gives credence to the concept that both phosphate and hydrogen phosphate units exist in the structure of lulzacite. The series of Raman bands at 567, 582, 601, 644, 661, 673 and 687 cm_1 are assigned to the PO3_ 4 m2 bending modes. The series of Raman bands at 437, 468, 478, 491, 503 cm_1 are attributed to the PO3_ 4 and HOPO2_ 3 m4 bending modes. No Raman bands of lulzacite which could be attributed to the hydroxyl stretching unit were observed. Infrared bands at 3511 and 3359 cm_1 are ascribed to the OH stretching vibration of the OH units. Very broad bands at 3022 and 3299 cm_1 are attributed to the OH stretching vibrations of water. Vibrational spectroscopy offers insights into the molecular structure of the phosphate mineral lulzacite.