Mineralogy of Pb-P Grains in the Roots of Agrostis capillaris L. by ATEM and EXAFS*

Mineralogy of Pb-P Grains in the Roots of Agrostis capillaris L. by ATEM and EXAFS^*

J. D. Cotter-Howells^{1, 2, †}, P. E. Champness¹ and J. M. Charnock^{1, 3}

¹ Department of Earth Sciences, The University of Manchester, Manchester M13 9PL, UK
² School of Biological Sciences, The University of Manchester, Manchester M13 9PL, UK
³ 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 18^th March 1997, entitled ‘Phosphates: biogenic to exotic’
^†Present address: Department of Plant and Soil Science, Cruickshank Building, Aberdeen University, Aberdeen AB24 3UU, UK.

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.

Mineralogical Magazine; December 1999 v. 63; no. 6; p. 777-789; DOI: 10.1180/002646199548880
© 1999, The Mineralogical Society
Mineralogical Society (www.minersoc.org)