Lead hydrogen citrate monohydrate, Pb(C6H6O7)·H2O, formation during specimen cleaning: a cautionary mineralogical tale

Abstract

AbstractCrystal-structure analysis was used to identify crystals of Pb(C6H6O7)·H2O on a specimen of hematite pseudomorphous after magnetite from the Payύn Matrύ volcanic field, Mendoza Province, Argentina. The compound is triclinic,P, a = 6.3434(2),b= 6.4566(2), c = 12.0640(9) Å, α = 99.233(7), β = 102.810(7), γ = 101.562(7)° andZ=2.The crystal structure, refined toR1= 0.0169 for 2021 reflections[Fo>4σ(F)],consists of a zig-zag chain of 9-coordinated Pb2+atoms alongb. Chains are linked together by citrate molecules to form thick irregular layers in theabplane. The layers are linked together only by H bonds. The citrate molecule exhibits normal bond lengths and angles. The Pb2+atom exhibits markedly lopsided coordination due to the 6s2lone-electron-pair effect. The lone electron pairs on alternating sets of adjacent Pb2+atoms in the chain point towards one another.The formation of the Pb(C6H6O7)·H2O crystals was the result of specimen cleaning in citric acid. The cleaning solution was apparently contaminated through the dissolution of minor associated Pb-bearing species by the citric acid. It is important to be aware of the possibility that micro-crystals may be inadvertently grown on mineral specimens during chemical cleaning, to carefully and critically inspect paragenetic relationships especially when studying mineral specimens that may have been cleaned in chemical agents, and to be particularly aware of health risks when treating Pb-bearing minerals in aqueous citric acid solutions.