gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/002646199548880 Mineralogy of Pb-P Grains in the Roots of Agrostis capillaris L. by ATEM and EXAFS* Cotter-Howells J. D. 1 2 † Champness P. E. 1 Charnock J. M. 1 3 Department of Earth Sciences, The University of Manchester, Manchester M13 9PL, UK School of Biological Sciences, The University of Manchester, Manchester M13 9PL, UK CLRC Daresbury Laboratory, Warrington WA4 4AD, UK

The first four papers in this issue were presented at the Mineralogical Society's Spring Meeting, held on 18th March 1997, entitled ‘Phosphates: biogenic to exotic’

Present address: Department of Plant and Soil Science, Cruickshank Building, Aberdeen University, Aberdeen AB24 3UU, UK.

 12 1999 63 6 777 789 8 9 1997 15 3 1999 Copyright © 1999, The Mineralogical Society 1999 Abstract

Analytical transmission electron microscopy (ATEM) and X-ray absorption spectroscopy (XAS) have been used to determine the mineralogy of Pb-P deposits in the roots of the heavy metal tolerant grass cultivar Agrostis capillaris L. cv. Parys Mountain. The deposits have a pyromorphite (Pb5(PO4)3Cl)-type structure and composition although some of the Cl may be substituted by OH. Energy-dispersive mapping under the scanning electron microscope demonstrated that the majority of these deposits are present in the outer cell wall of the epidermis (the outermost layer of root cells). The phosphate composition of these grains contrasts with the phytate (C6H18O24P612−) composition of Zn-P deposits observed in similar electron microscopy studies. The physiological role of heavy metal P deposits is unclear. Heavy metal P precipitates may form actively as a tolerance mechanism to heavy metals or passively, sequestering P in a metabolically inactive form.

 Keywords lead lead phosphate pyromorphite lead hydroxyapatite electron microscopy electron diffraction heavy metal tolerance Agrostis capillaris