Crystal chemistry of synthetic pyromorphite-mimetite solid solutions

Abstract

Pyromorphite Pb 5 [PO 4 ] 3 Cl, mimetite Pb 5 [AsO 4 ] 3 Cl, and their solid solutions were synthesized from aqueous solutions. These minerals form a ternary system within apatite group of P6 3 /m symmetry (hexagonal bipyramid). Natural pyromorphite usually contains several percent of As. Also, natural mimetite often contains PO 4 substitutions. The objective of this study was determination of regularities in crystalline lattice parameters changes with anionic substitution of  and . The products were characterized using X-ray diffraction and scanning electron microscopy with microanalysis. The synthesis resulted in fine white precipitate. X-ray diffraction indicated crystalline character of the precipitate. Lattice parameters for end-members are in accordance with ICDD standards and equal for pyromorphite a = 10,009 Å and c = 7,338 Å and for mimetite a = 10,247 Å and c = 7,448 Å. Both lattice parameters increase proportionally to the increase of As substitution in pyromorphite in conformation with Vegard’s law.