Experimental investigation of the mechanism of the reaction: 1 tremolite+11 dolomite ? 8 forsterite+13 calcite+9 CO2+1H2O |
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| Authors: | Wilhelm Heinrich Paul Metz Werner Bayh |
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| Institution: | 1. Facultad de Ciencias de La Tierra, Universidad autónoma de Nuevo León, Apartado postal 104, 67700, Linares, N.L., Mexico
2. Mineralogisch-Petrographisches Institut der Universit?t Tübingen, Wilhelmstrasse 56, D-7400, Tübingen, Federal Republic of Germany
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| Abstract: | Stoichiometric mixtures of tremolite and dolomite were heated to 50° C above equilibrium temperatures to form forsterite and calcite. The pressure of the CO2-H2O fluid was 5 Kb and \(X_{{\text{CO}}_{\text{2}} }\) varied from 0.1 to 0.6. The extent of the conversion was determined by the amount of CO2 produced. The resulting mixtures of unreacted tremolite and dolomite and of newly-formed forsterite and calcite were examined with a scanning electron microscope. All tremolite and dolomite grains showed obvious signs of dissolution. At fluid compositions with \(X_{{\text{CO}}_{\text{2}} }\) less than about 0.4, the forsterite and calcite crystals are randomly distributed throughout the charges, indicating that surfaces of the reactants are not a controlling factor with respect to the sites of nucleation of the products. A change is observed when \(X_{{\text{CO}}_{\text{2}} }\) is greater than about 0.4; the forsterite and calcite crystals now nucleate and grow at the surface of the dolomite grains, thus indicating a change in mechanism at medium CO2 concentrations. As the reaction progresses, the dolomite grains become more and more surrounded by forsterite and calcite, finally forming armoured relics of dolomite. Under experimental conditions this characteristic texture can only be formed if the CO2-concentration is greater than about 40 mole %. These findings make it possible to estimate the CO2-concentration from the texture of the dolomite+tremolite+forsterite+calcite assemblage. The results suggest a dissolution-precipitation mechanism for the reaction investigated. In a simplified form it consists of the following 4 steps: - Dissolution of the reactants tremolite and dolomite.
- Diffusion of the dissolved constituents in the fluid.
- Heterogeneous nucleation of the product minerals.
- Growth of forsterite and calcite from the fluid.
Two possible explanations are discussed for the development of the amoured texture at \(X_{{\text{CO}}_{\text{2}} }\) above 0.4. The first is based upon the assumption that dolomite has a lower rate of dissolution than tremolite at high \(X_{{\text{CO}}_{\text{2}} }\) values resulting in preferential calcite and forsterite nucleation and growth on the dolomite surface. An alternative explanation is the formation of a raised CO2 concentration around the dolomite grains at high \(X_{{\text{CO}}_{\text{2}} }\) values, leading to product precipitation on the dolomite crystals. |
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