A combination of atomic force microscopy, scanning election microscopy, transmission electron microscopy, energy dispersive spectroscopy, electron diffraction, and X-ray diffraction were used to study reactions of 0.5-500 mg/L aqueous Pb with Ca 5 (PO 4) 3 OH, hydroxylapatite (HAP), at pH 6 and 22 ЊC. Following 2 h reaction time, concentrations of Pb aq ([Pb aq ]) decreased from 500 mg/L to Ͻ100 mg/L, and from 0.5-100 mg/L to Ͻ15 g/L. This loss of Pb aq from solution (i.e., sorption) resulted partially from simultaneous dissolution of HAP and precipitation of Pb 5 (PO 4) 3 OH, hydroxypyromorphite (HPY), or another solid Pb phase. The initial saturation state with respect to HPY (defined as the ratio of the ion activity product to equilibrium solubility product) influenced strongly precipitation processes. At a high degree of saturation (initial [Pb aq ] Ͼ 100 mg/L), small nuclei or aggregates of poorly crystalline HPY precipitated homogeneously in solution. At intermediate saturation (initial [Pb aq ] ϳ10-100 mg/L), large, euhedral needles of HPY precipitated homogeneously in solution. At a low degree of saturation (initial [Pb aq ] Ͻ 10 mg/L), a needle-like Pb-containing phase grew heterogeneously on HAP. These results agree well with concepts derived from nucleation and growth theories and demonstrate that initial saturation state influences strongly the sorption process.

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The middle Permian Chihsia Formation of south China accumulated on a shallow shelf, and consists mainly of black to dark grey micritic limestone rich in chert nodules and organic matter. A unique type of nodular crystal cluster is distributed widely in the carbonate succession. Most crystal clusters consist of calcite. Some, however, are composed of celestite, and geochemical, microscopic and crystal morphological data suggest that celestite was the precursor of the calcite. The celestite developed displacively within the sediments during early diagenesis, before compaction and before local dolomitization of the host rock. Similar strontium isotopic values were obtained from the celestite clusters, replacement calcite, vein calcite and host rock. The values are within the range of middle Permian sea water. The strontium in the celestite was furnished chiefly by either diagenetic alteration of strontium‐rich marine aragonite to strontium‐poor calcite, or aragonite dissolution induced by aerobic oxidation of organic matter, or both. The sulphur isotopic values of the celestite are about 6–11‰ heavier than the sulphur isotopic value of sulphate in coeval sea water. Based on geological context, this difference is attributed to microbial reduction of porewater sulphate in the Chihsia sediments.

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