Synthesis and reactivity of minerals of dolomite group and analogue phases (Síntesis y reactividad de minerales del grupo de la dolomita y fases análogas)


Dolomite, CaMg(CO3)2, is a calcium magnesium carbonate mineral ubiquitous in the earth crust. Dolomite is the second most abundant carbonate mineral after calcite, CaCO3. Since its first description in 1791, a number of different unsuccessful methodologies has been used to tried to synthesize dolomite in the laboratory at ambient pressure and temperature. In mineralogy, this is known as the “dolomite problem”. This Ph.D. thesis provides a new contribution to the future resolution of the dolomite problem. The thesis comprises: (I) a study of the formation process of dolomite and dolomite analogue phases at ambient temperature and pressure and (II) a study of the reactivity of (10.4) dolomite and kutnohorite (CaMn(CO3)2) surfaces in contact with supersaturated aqueous solution with respect to various monocationic carbonates (i. e. calcite [CaCO3], otavite [CdCO3], sphaerocobaltite [CoCO3] and zabuyelite [Li2CO3]). The study of the processes leading to the formation of dolomite-like phases and dolomite analogue phases was carried out using the following methodologies: (I) mixing aqueous solutions, (II) ageing of suspended precursor phases in aqueous solutions, (III) ageing of precursor phases in solutions with acidification – basification cycles and (IV) precipitating phases using seawater and acidification – basification cycles in presence of additives. By the ageing of the precipitates previously obtained by mixing aqueous solutions the synthesis of norsethite (BaMg(CO3)2) and PbMg(CO3)2 with cationic ordering was achieved in less than 10 days. Using the same method, the phases CaMg(CO3)2, BaCa(CO3)2 and CdMg(CO3)2 without cationic ordering were also synthesized. Alternatively, norsethite was synthesized by the ageing of precursor phases suspended in aqueous solutions. However, the time required in these experiments of synthesis of norsethite was longer than that in the experiments by mixing solutions. The synthesis of dolomite was not achieved by using any of the four methodologies mentioned above. The reactivity experiments on the (10.4) dolomite and kutnohorite surfaces was conducted by promoting the growth of different phases (i. e. calcite, otavite, sphaerocobaltite and zabuyelite) on that surfaces. These experiments were carried out using an atomic force microscopy (AFM). A nanotribological study was also performed on the substrates and overgrowths surfaces. The calcite, otavite, sphaerocobaltite and zabuyelite overgrowths showed a different formation behaviour. In the case of calcite, otavite and sphaerocobaltite, the growth was epitaxial. The nanotribological study of the overgrowths consisted in both the quantification of the adhesion between the overgrowth and the substrate (calcite on dolomite and on kutnohorite) and the quantification of the frictional forces between the AFM tip and the overgrowths or the substrates. The different frictional response observed in the overgrowths and the substrates allowed us to distinguish them quickly. The main conclusions drawn from this thesis are: (I) The formation and cationic ordering processes that lead to the crystallization of the dolomite like phases and the dolomite analogues have different kinetics, mainly depending on the ratios of cationic radii (cation2+:Mg2+): the larger the ratios, the fastest is the process. (II) The experimental results suggest that the hydration of Mg2+ is not the main factor that inhibits the formation of dolomite at ambient temperature in the laboratory. Since norsethite and PbMg(CO3)2 were synthesized under ambient conditions, and the formation of both phases required (as for dolomite) an equal incorporation of Mg2+ and the other cations (i. e. Ba2+ and Pb2+) in their structures, we can conclude that dehydration of Mg2+ is not a significant rate-limiting factor. (III) Different reaction pathways towards the formation of norsethite and PbMg(CO3)2 were identified. Moreover, the precursor phases of such dolomite analogue phases, as well as their evolution, were described. (IV) The growth of monocationic carbonates on the (10.4) dolomite and kutnohorite surfaces is mainly controlled by the lattice misfits between the overgrowths and the substrates. Calcite, otavite and sphaerocobaltite grew epitaxially on the substrates. This was verified by the analysis of the high resolution AFM images of the overgrowth and substrate surfaces, in which the crystalline structures of the monocationic carbonate of the overgrowths resulted to be parallel to the structure of the substrates. When zabuyelite highly supersaturated solutions were used, the growth of zabuyelite on these substrates is epitaxial. However, when zabuyelite slightly supersaturated or subsaturated solutions were used, the growth mechanism could not be identified, because high resolution AFM images were not obtained on those overgrowths, due to (I) there is no zabuyelite surfaces parallel to the substrate surfaces or (II) the growing phase is an amorphous phase. (V) On the (10.4) dolomite and kutnohorite surfaces, the scanning with the AFM tip affects the nucleation of the phases (i. e. calcite and zabuyelite). While the nucleation of calcite islands was inhibited by the scanning, the nucleation of zabuyalite was favored by it. (VI) The nanotribological study of the overgrowths indicates that the different overgrowths show different frictional responses. While otavite and sphaerocobaltite have a higher friction coefficient than the substrates on which they grow, zabuyelite has a lower friction coefficient than such substrates. Through nanomanipulation experiments conducted on the (10.4) dolomite and kutnohorite surfaces, the minima shear strength required for detaching calcite islands from such surfaces were estimated. The shear strength between calcite and kutnohorite is larger than that between calcite and dolomite. (VII) In summary, the processes leading to the formation of a number of dolomite analogue phases at ambient temperature were determined and monitorized. Moreover, the crystal growth of some monocationic carbonates on the surfaces of two minerals with dolomite structure were investigated. The results presented in thesis can be considered as a starting point for future works on the crystallization of dolomite and dolomite analogue phases at ambient conditions, and for investigating crystal growth phenomena on the surfaces of double carbonates.