International Geologiical Congress - Oslo 2008

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MPE-01 General contributions to experimental petrology and mineralogy in honour of Professor Surendra Saxena

 

Experimental study of dissolution rates of silicate minerals in water up to 435 ° C at pressures of 23 to 33MPa

 

Xuetong Zhang, Institute of Mineral Resources, Chinese Academy of Geological Sciences (China)
Ronghua Zhang, Institute of Mineral Resources, Chinese Academy of Geological Sciences (China)
Shumin Hu, Institute of Mineral Resources, Chinese Academy of Geological Sciences (China)
 

 

Dissolution rates of silicate minerals (albite Ab, actinolite Act, diopside Di, hedenbergite Hd and quartz Q) in water were measured using a flow-through reactor in the temperature range from 25 to 435°C and at pressures of 22 to 33 MPa. Different metals possess generally non-stoichiometric release rate. Results show that metals (Na, Ca, Mg, Fe, Al) dissolve faster than Si at temperatures of 25 to 300°C, but slower at temperatures of 300 to 400°C at 23MPa. The dissolution stoichiometry becomes nearly congruent at 300°C (e.g. Ab, Hd, Act etc.).
Usually, the release rates of metal Mk (Mk = Si, Al, Na, Ca, Fe, Mg, etc.) in the minerals (Ab, Act, Di, Hd) increase with increasing temperature from 20 to 300°C. But all of the rates decreased with increasing temperature from 300 to 400°C. The maximum release rates of Si in minerals in water are often present at 300°C 23MPa.
If a multi-oxide silicate dissolution is nearly congruent, one could determine its rate law. Experiments prove that the logarithm of the far from equilibrium dissolution rates (release rates) of the mineral will be linearly related to the logarithm of the activity of dissolved Mk. Usually, its far from equilibrium dissolution rates at all pH of the solution would depend on both pH and aqueous Mk Zk activity. Zk is the valence of the metal Mk in the mineral. As a multi-oxide silicate mineral dissolves in water, a metal Mk is removed from the near surface, released to solution, potentially leading to the formation of a Mk depleted leached layer on surface. The relative rates at which various metal-oxygen bonds are broken within a multi-oxide structure are different. The initial preferential release of a metal is both univalent and divalent cation, and then alumi-num, finally Si. Thus, the release rate ratios ri/rSi decrease with increasing temperature from 20 to 400°C. Within and near the critical region, the rapid decreases in the dielectric constant and density of water re-sult in the destruction of the hydrogen bond network of the water molecules, thus, both the ionic and hydra-tion reactions are weakening. However, owing to differences in polarizability among the Ca-Obr (bridge oxygen), Mg-Obr, Al-Obr and Si-Obr bonds, the Si-Obr bond is more easily hydrated at temperatures >300°C. The maximum dissolution rate of quartz occurs at 374°C 23MPa. As pressure increases to 33MPa, the maximum dissolution rate is present at 400°C. With continued increasing temperature, dissolution rates of quartz decrease at T>374°C (23MPa) and at T>400°C (33MPa). Previous experiments on dissolution rates of albite and quartz were performed at 300°C and at pressures of 12MPa (<23MPa). Those rate values at 300°C (12MPa) are lower than the values of this study at 300°C (23 to 33MPa).

 

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