RAMAN SPECTROSCOPY OF PHOSPHATES OF THE VARISCITE MINERAL GROUP I: IMPLICATIONS FOR SOIL CHEMISTRY

Abstract: This study reports on Raman and IR spectra of the variscite phosphate minerals and relates the vibrational spectra to the mineral structure. The application of the spectroscopy is in the possible determination of colloidal variscite minerals in soils. The variscite mineral group are orthorhombic arsenates and phosphate with the general formula AXO4.2H2O, where A = Al3+, Fe3+, Cr3+ or In3+ and X = As or P, with space group Pcab. The most common of the phosphate minerals are strengite (Fe3+ PO4.2H2O) and variscite (AlPO4.2H2O) [1], with metavariscite, metastrengite and phosphosiderite (also known as clinostrengite) less common. Strengite forms a series with variscite and is dimorphous with metavariscite. Variscite is dimorphous with metavariscite [2, 3]. Metavariscite (AlPO4.2H2O) is monoclinic with the space group P21/n. The formation of variscite group minerals in soils is of importance. It is probable that colloidal variscite and strengite are important in the uptake of phosphorus by plants [4, 5]. Such minerals can be formed through the weathering of phosphate rocks but a more likely to be formed through the addition of rock phosphates to soils. Raman spectroscopy of phosphates of the variscite mineral group namely strengite, variscite, metavariscite and phosphosiderite have been obtained at 298 K and 77 K using a thermal stage. The Raman data was complimented by selected infrared data. Multiple phosphate stretching band