Experimental study of the replacement of calcite by calcium sulphates

Microscopic Studies of Calcium Sulfate Crystallization and Transformation at Aqueous–Organic Interfaces

“…However, the formation of the gypsum layer passivates calcite and only a thin surface layer of gypsum is formed. 20 It is likely that an analogous mineral replacement reaction takes place in our system, but the process goes to completion because the parent phase in our system is in the form of nanoparticles, and a large percentage of the CaCO3 is present at the nanoparticle surface and is thus highly accessible to the surrounding SO4 2--containing fluid. The hydrated CaSO4 produced is the precursor to the gypsum that grows as needles in the aqueous phase and, ultimately, within the oil phase due to stabilization of the nanoparticles by the hydrophobic alkyl sulfonate surfactant.…”

“…Figure 3a shows that the crystals analyzed gave a strong peak centered at 1008 cm -1 , which corresponds to the ν1 SO4 shift for gypsum (CaSO4⋅2H2O), which has been previously reported at 1008 -1010 cm -1 . 20,[34][35][36] Due to the higher degree of hydration in the crystal structure, this peak occurs at lower wavenumbers than for bassanite (CaSO4⋅0.5H2O), present at 1015 -1016 cm -1 , 20,35,36 and anhydrite (CaSO4), at 1017 -1018 cm -1 . 20,[34][35][36] This is therefore a definitive way of deducing the calcium sulfate phase grown.…”

“…37 Thus, for the system employed in our work, it is plausible that the presence of the H2SO4 droplet results in the Interestingly, the pseudomorphic replacement of calcite by an epitaxial layer of CaSO4 (either gypsum, bassanite or anhydrite depending on the temperature of the reaction) has recently been reported. 20 This was achieved by part-dissolving millimeter-sized calcite pieces in H2SO4.…”

“…[12][13][14] The most thermodynamically stable phase of calcium sulfate at room temperature is the dihydrate, gypsum (CaSO4⋅2H2O) and, as a consequence, it is highly abundant in nature. 11,[15][16][17][18][19][20][21][22] Both the hemihydrate, bassanite (CaSO4⋅0.5H2O) and the anhydrous form, anhydrite (CaSO4) are relatively thermodynamically unstable and kinetically unfavorable under ambient conditions, but each has important industrial applications. Bassanite (more commonly known as plaster of Paris), is a highly valuable building material, whereas anhydrite has applications as a binder in cements and adhesives.…”

“…15,18,[20][21][22][23][24][25] There is much debate as to the transition temperature. Previous reported values under atmospheric conditions generally fall within the broad range of 42 -60 °C, 15,26 although in practice, it is extremely difficult to synthesize anhydrite at temperatures lower than 70 °C.…”