Synthesis of zoning-free BaSO₄-PbSO₄ solid solution and its structural characterizations

Abstract

Natural Pb-bearing barite, (Ba,Pb)SO₄ “hokutolite”, is a radioactive sulfate mineral occurring in hot-spring deposit. An attempt to synthesize (Ba,Pb)SO₄ crystal with a flux-growth process was made in this study. BaSO₄ and PbSO₄ in various proportions were used as precursor materials with Na₂SO₄ as a flux agent. The growth was carried out in a platinum crucible at 750-850ºC and ambient pressure conditions. The nucleation and growth were activated by a slow-cooling process at 2ºC/hr cooling rate. The flux-grown (Ba,Pb)SO₄ crystals up to 1 mm in dimension were obtained and their chemical compositions were found to cover the entire range between BaSO₄ and PbSO₄. Thus there does exist a complete solid solution series between the barite (BaSO₄) and anglesite (PbSO₄) binary system. This result is consistent with that reported for (Ba,Pb)SO₄ synthesized with a hydrothermal method. Both natural and hydrothermally synthesized (Ba,Pb)SO₄ crystals display a distinct domain structure in which the Ba/Pb ratio varied among different domains. This type of chemical zoning is recognized under an optical microscope to be a banded texture in which dark bands (Ba-rich area) alternate with bright bands (Pb-rich area). In contrast, the (Ba,Pb)SO₄ samples synthesized in this work contain no detectable optical bands, and this dramatic difference is believed to be a direct consequence of different growth kinetics. The flux-grown (Ba,Pb)SO₄ crystals were determined to be a barite type of structure with the Ba and Pb atoms substituting each other in a fully disordered arrangement. The cell parameters vary continuously and systematically from the Ba-rich end toward the Pb-rich end as dictated by an ideal solid solution series.