Kyanite far from equilibrium dissolution rate at 0–22 °C and pH of 3.5–7.5

Abstract

Kyanite is an important and slow-dissolving mineral. Earlier work has measured its dissolution rate at high temperature and acidic pH, but experimental measurements at low temperature and near neutral pH were lacking. The rate equation by Palandri and Kharaka (A compilation of rate parameters of water–mineral interaction kinetics for application to geochemical modeling. US Geological Survey, Open File Report 2004-1068, 2004) indicates that the rate of kyanite dissolution at room temperature and near neutral pH is on the order of 10⁻¹⁷ mol m⁻² s⁻¹, orders of magnitudes slower than most common silicate minerals such as albite and quartz. This study used an externally-stirred mixed-flow reactor, which allows high solid:solution ratios, to measure the dissolution rate of kyanite at 0–22 °C and pH of 3.5–7.5. The measured dissolution rate of kyanite is 4.6–7.6 × 10⁻¹³ mol m⁻² s⁻¹ at 22 °C, and the apparent activation energy is 73.5 kJ mol⁻¹. This dissolution rate is close to the rate of quartz dissolution and four orders of magnitude faster than the prediction by rate equation of Palandri and Kharaka (2004).

Based on our new experimental data, we recommend the following rate equation for modeling the dissolution of kyanite at ambient temperatures.

$$r = ke^{{\frac{{ - E_{a} }}{R}\left( {\frac{1}{T} - \frac{1}{298.15}}\right)}}$$

where k = 5.08 × 10⁻¹³ mol m⁻² s⁻¹, and E_a = 73.5 kJ mol⁻¹. Review of literature data (Carroll in The dissolution behavior of corundum, kaolinite, and andalusite: a surface complex reaction model for the dissolution of aluminosilicate minerals in diagenetic and weathering environs. Dissertation, Northwestern University, 1989) led to a recommended rate equation for andalusite as for T = 25 °C and pH = 2–10:

$$r = k_{1} a_{{{\text{H}}^{ + } }}^{{n_{1} }} + k_{2} + k_{3} a_{{{\text{H}}^{ + } }}^{{n_{3} }}$$

where k₁ = 4.04 × 10⁻¹⁰ mol m⁻² s⁻¹, k₂ = 7.95 × 10⁻¹⁰ mol m⁻² s⁻¹, k₃ = 1.01 × 10⁻¹⁷ mol m⁻² s⁻¹, n₁ = 1.2 and n₃ = − 0.6.