Crystal chemistry of synthetic pyromorphite-mimetite solid solutions

Abstract

Pyromorphite Pb₅[PO₄]₃Cl, mimetite Pb₅[AsO₄]₃Cl, and their solid solutions were synthesized from aqueous solutions. These minerals form a ternary system within apatite group of Pb₃/m symmetry (hexagonal bipyramid). Natural pyromorphite usually contains several percent of As. Also, natural mimetite often contains PO₄ substitutions. The objective of this study was determination of regularities in crystalline lattice parameters changes with anionic substitution of AsO₄^-3 and PO₄^-3. The products were characterized using X-ray diffraction and scanning electron microscopy with micro-analysis. The synthesis yielded a fine white precipitate. X-ray diffraction indicated crystalline character of the precipitate. The 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=10,247Å and c=7,448 Å. Both lattice parameters increase proportionally to the increase of As substitution in pyromorphite in conformation with Vegard’s law.