Abstract

Hedyphane, traditionally formulated as (Ca,Pb)5(AsO4,PO4)3Cl, is redefined as Ca2Pb3(AsO4)3Cl with Z = 2. The space group is P63/m with a = 10.140(3) and c = 7.185(4)Å. Observed and calculated densities are 5.85 and 5.99 g/cm2, respectively. The crystal structure has been refined to residuals of 0.062 (weighted) and 0.076 (unweighted) using 550 reflections. Ordering of Ca and Pb on equipoints 4f and 6h, respectively, is complete except for a possible minor amount of solid solution (ca. 1%). Cation ordering in hedyphane is attributed to a preference by atoms with sterically active electron lone pairs, e.g., Pb(II), for the asymmetrically coordinated 6h site. A study of minerals in the solid solution series CaxPb5−x(AsO4)3Cl from Långban, Sweden, and Franklin, New Jersey, reveals a lack of compositions in the range 2.3 ≤ x ≤ 4.8, implying a miscibility gap in this region.

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