The PbCrO4-PbSO4 system and its mineralogical significance

X-ray powder diffraction studies have shown that solid solution in the orthorhombic lead(II) chromate-orthorhombic lead(II) sulfate system, space group Pnma is lacunar at 25ºC, with the miscibility gap bounded by Pb(Cr04)o.8(S04)o.2 and Pb(Cr04)o.1(S04)o.9. All Cr-rich compositions are metastable with respect to the monoclinic polymorph, corresponding to crocoite, space group P21 /n. The extent of solid solution of sulfate in crocoite at 25ºC is Pb(Cr04)O.6(S04)O.4. The transformation of orthorhombic Pb(Cr04)x(S04)I-x to the monoclinic phase is slow in the solid state, as compared to when solid samples are allowed to remain in contact with the solution from which they crystallized. Recrystallization at ambient temperatures is acid-catalyzed and these effects explain a number of disparate reports concerning the stabilities of the various series in the literature. A value for the solubility product of orthorhombic lead(II) chromate has been determined using solution methods as logKsp(PbCr04,s, orthorhombic, 298.2 K) = -10.71 (12) and this has been used in turn to assess the nature of solid solution in Pb(Cr041.JS04)I_iO<x