The pseudomorphic replacement of marble by apatite: The role of fluid composition

•The hydrothermal replacement of Carrara marble (calcite) by apatite for different fluid compositions is presented•The replacement is pseudomorphic•The replacement rates decreased in the presence of high Cl concentrations whilst in the presence of F increased•Small fluid compositional changes can change the nature and the microstructure of rocks•Mass transfer through grain boundaries becomes more significant for the case of lower replacement rates

The replacement of a natural carbonate rock (Carrara marble) by apatite was used as a model to study the role of fluid chemistry in replacement reactions, focusing on the mineralogy, chemical composition, and porosity of the replacement product. Carrara marble was reacted with diammonium phosphate solutions ((NH4)2HPO4), in the presence and absence of four salt solutions (NH4Cl, NaCl, NH4F, and NaF) at different ionic strengths, at 200 °C and autogenous pressure. The replacement products were analyzed using powder X-ray diffraction, Scanning electron microscopy (SEM), electron microprobe analysis (EMPA), and Raman spectroscopy. The reaction in all samples resulted in pseudomorphic replacements and shared the characteristics of an interface-coupled dissolution-precipitation mechanism. Increasing the ionic strength of the phosphate fluid increased the replacement rates. With a fixed concentration of phosphate, replacement rates were reduced with the addition of NH4Cl and NaCl and increased significantly with the addition of NaF and NH4F. The addition of different salts resulted in specific porosity structures resulting from the formation of different phosphate phases. Chloride-containing fluids showed a higher degree of fluid percolation through grain boundaries. This study illustrates the significant impact that small differences in solvent composition can have in the progress of replacement reactions, the nature of the products and the resultant porosity.