Baryte cohesive layers formed on a (010) gypsum surface by a pseudomorphic replacement

Abstract

The mineral replacement of gypsum (CaSO4·2H2O) by baryte (BaSO4) is relevant to technological and industrial applications, including its use as a plaster or stone consolidant in cultural heritage conservation. In the present study, we provide experimental evidence suggesting that, during the interaction of gypsum cleavage surfaces with barium-bearing solutions, a pseudomorphic replacement takes place and results in the formation of a crystallographically oriented baryte layer. This mineral replacement process is favoured by the porosity generated, due to the differences in molar volume and solubility between parent and product sulfate phases, allowing the progress of the reaction. The homogeneous micrometre-sized layer of baryte occurs most likely via a fluid-mediated interface-coupled dissolution–precipitation mechanism. A certain degree of crystallographic control on the polycrystalline BaSO4 product layer by the structure of the parent substrate (gypsum) is confirmed by electron microscopy observations and X-ray diffraction analyses. The structural control exerted by the cleavage gypsum surface on the baryte layer can be defined by the epitactic relationship: Gyp (010) || Bar (010). The formation of baryte increases with reaction time until passivation occurs at the replacement interface, probably due to a decreased porosity and loss of connectivity that thereby prevents further reaction. The investigation of these processes occurring on freshly cleaved single crystals of gypsum were complemented by studying the replacement of polycrystalline gypsum cubes, showing a homogeneous baryte surface layer on the sample. The results of this study thus offer interesting insights into the application of the replacement of gypsum by baryte as a conservation method for gypsum sculptures and plasterwork, increasing their resistance against water and humidity while preserving the surface features of the original mineral substrate.