Taylor et al.(2000)による〔『Kinetics of dissolution and Sr release during biotite and phlogopite weathering』(1191p)から〕


【フロー型反応器(flow-through column reactor)による黒雲母および金雲母の溶解実験】

 The dissolution kinetics and isotopic composition of Sr released during the weathering of biotite and phlogopite were measured under experimental conditions at pH 〜 3 and 25゜C. Although the overall release of Sr was essentially stoichiometric, the 87Sr/86Sr ratios of weathering solutions were generally higher than the reacting mineral. The initial phase of dissolution was characterized by the simultaneous rapid release of Sr from the sheet silicate and from trace amounts of calcite inclusions. During the initial phase, the 87Sr/86Sr ratios of the output solutions were lower than either the bulk mineral or the later, steady-state ratios. After the early stages of the experiments, calcite dissolution became limited by the rate at which it was exposed to the weathering solution and the 87Sr/86Sr of the output solutions increased to values above those of the biotite and phlogopite (after correction for the calcite inclusions). Differences in the location of Sr and Rb within the mica structure may cause the Sr isotopic ratio of the effluent to increase above the mineral ratio. The log values of the steady-state Sr release rate constants (moles per meter per second) were determined to be -15.1 for biotite and -15.9 for phlogopite under our experimental conditions. Cation release rates indicate that different sites within the sheet silicate structure reacted at different rates. Interlayer cations were generally released more rapidly than octahedral cations, which reacted slightly faster than the tetrahedral cations. Interlayer potassium release was considered to be controlled by diffusion with a moving boundary condition. Diffusion coefficients calculated from this model were 3.5×10-19 cm2/s for biotite and 1.9×10-18 cm2/s for phlogopite. Iron oxidation appeared to limit the overall biotite dissolution rate. Iron-free phlogopite reacted nearly twice as fast as biotite. The log values of the overall dissolution rates (moles per meter per second) were determined to be -11.6 for biotite and -11.2 for phlogopite. Cation release during phlogopite dissolution was also much less stoichiometric than during biotite weathering.』

 黒雲母と金雲母が風化する間の、溶解カイネティックスと放出されるSrの同位体組成が、25℃およびpH〜3の実験条件下で測定された。Sr全体の放出は本質的に化学量論的であるが、風化溶液の87Sr/86Sr 比は一般に反応鉱物よりも高かった。溶解の初期段階は、シート珪酸塩および微量の方解石包有物から同時に生じたSrの急速な放出により、特徴づけられた。初期段階の間、流出した溶液の87Sr/86Sr 比は、バルクの鉱物あるいは後期の定常状態での比の、どちらに比べても低かった。実験の初期段階の後、方解石の溶解は、風化溶液に露出していた時の速度により制限されるようなり、流出した溶液の87Sr/86Sr は、黒雲母と金雲母の値以上(方解石包有物の補正後)に増加した。雲母の構造内部のSrとRbの配置の違いのため、流出液のSr同位体比は、鉱物自身の値以上に増えることになるだろう。定常状態のSr放出速度定数(モル/m2/秒)の対数値は、我々の実験条件で、黒雲母は-15.1、金雲母は-15.9と決定された。陽イオン放出速度の結果は、シート珪酸塩構造内部の異なるサイトでは異なる速度で反応したことを示している。層間陽イオンは、一般に八面体陽イオンより速く放出され、八面体陽イオンは四面体陽イオンよりわずかに速く反応した。層間のカリウムの放出は、移動境界条件をもつ拡散によってコントロールされると考えた。このモデルから計算された拡散係数は、黒雲母が3.5×10-19 cm2/秒、金雲母が1.9×10-18 cm2/秒であった。鉄の酸化は、黒雲母の全体の溶解速度に制限を与えると思われた。鉄を含まない金雲母は、黒雲母のほぼ2倍近い速さで反応した。全体の溶解速度(モル/m2/秒)の対数値は、黒雲母が-11.6、金雲母が-11.2と決定された。金雲母が溶解する間の陽イオン放出も、黒雲母が風化する間より、化学量論性がかなり小さかった。』

1. Introduction
2. Methods

 2.1. Reactor design
 2.2. Characterization of starting material
 2.3. Experimental procedure
 2.4. Analytical methods
3. Results and discussion
 3.1. Elemental concentrations in output solutions
 3.2. Sr isotopic ratios in output solutions
 3.3. Determining dissolution rates using column reactors
 3.4. Stoichiometry of weathering
 3.5. Calculating biotite and phlogopite dissolution rates
 3.6. Dissolution rates normalized to edge surface area
 3.7. Coupled diffusion and dissolution model: K release
 3.8. Comparison of biotite and phlogopite weathering rates
 3.9. Calcite contribution to experimental Sr release
 3.10. Sr release from biotite
 3.11. Isotopic evolution of biotite in a weathering environment
4. Conclusions