『Abstract
The present study compares the dissolution rates of plagioclase,
microcline and biotite/chlorite from a bulk granite to the dissolution
rates of the same minerals in mineral-rich fractions that were
separated from the granite sample. The dissolution rate of plagioclase
is enhanced with time as a result of exposure of its surface sites
due to the removal of an iron oxide coating. Removal of the iron
coating was slower in the experiment with the bulk granite than
in the mineral-rich fractions due to a higher Fe concentration
from biotite dissolution. As a result, the increase in plagioclase
dissolution rate was initially slower in the experiment with the
bulk granite. The measured steady state dissolution rates of both
plagioclase (6.2±1.2×10-11 mol g-1 s-1)
and microcline (1.6±0.3×10-11 mol g-1 s-1)
were the same in experiments conducted with the plagioclase-rich
fraction, the alkali feldspar-rich fraction and the bulk granite.
Based on the observed release rates of the major elements, we
suggest that the biotite/chlorite-rich fraction dissolved non-congruently
under near-equilibrium conditions. In contrast, the biotite and
chlorite within the bulk granite sample dissolved congruently
under far from equilibrium conditions. These differences result
from variations in the degree of saturation of the solutions with
respect to both the dissolving biotite/chlorite and to nontronite,
which probably was precipitating during dissolution of the biotite
and chlorite-rich fraction. Following drying of the bulk granite,
the dissolution rate of biotite was significantly enhanced, whereas
the dissolution rate of plagioclase decreased.
The presence of coatings, wetting and drying cycles and near
equilibrium conditions all significantly affect mineral dissolution
rates in the field in comparison to the dissolution rate of fully
wetted clean minerals under far from equilibrium laboratory conditions.
To bridge the gap between the field and the laboratory mineral
dissolution rates, these effects on dissolution rate should be
further studied.』
『要旨
本研究は、花崗岩全岩からの斜長石・マイクロクリン（微斜カリ長石）・黒雲母／緑泥石の溶解速度を、花崗岩試料から分離した鉱物片の同一鉱物の溶解速度と比較している。斜長石の溶解速度は、鉄酸化物被膜が取り去られることでその表面サイトが露出する結果として時間とともに大きくなる。鉄被膜の除去は、黒雲母の溶解によってFe濃度が高くなるため鉱物片よりも花崗岩全岩での実験の方で遅かった。その結果、斜長石溶解速度の増加は、花崗岩全岩では実験の最初に遅かった。斜長石（6.2±1.2×10-11モル/g/秒）とマイクロクリン（1.6±0.3×10-11モル/g/秒）の両方について測定された定常状態溶解速度は、斜長石鉱物片・アルカリ長石鉱物片・花崗岩全岩で行われた実験では同じだった。
主要元素で観察された放出速度に基づいて、平衡に近い条件で黒雲母／緑泥石鉱物片はインコングリュエント（不一致）に溶解することを我々は示している。対照的に、花崗岩全岩試料の黒雲母と緑泥石は平衡から離れた条件でコングリュエント（一致）に溶解した。これらの差は、溶解している黒雲母／緑泥石の両者および、黒雲母と緑泥石鉱物片の溶解中におそらく沈殿したノントロナイトに対して、溶液の飽和度が変動したために生じている。花崗岩全岩を乾燥した後では、黒雲母の溶解速度ははっきりと増大し、一方斜長石の溶解速度は減少した。
被膜の存在・乾湿サイクル・平衡に近い条件は、平衡から離れた実験室条件下での完全に水につけた純粋な鉱物の溶解速度と比較して、野外での鉱物溶解速度にどれも大きな影響を与える。野外と実験室での鉱物溶解速度の相違をに橋渡しをするには、溶解速度に対するこれらの影響をさらに研究する必要がある。』
1. Introduction
2. Materials and methods
3. Calculations
4. Results
4.1. Chemistry of bulk granite and the mineral-rich fractions
4.2. Dissolution experiments
5. Discussion
5.1. Chemical and mineralogical composition of the bulk granite
and the mineral-rich fractions
5.2. The release rate of major elements from the plagioclase-
and alkali feldspar-rich fractions
5.3. The effect of iron coating on the dissolution rate of plagioclase
5.4. The release rates of major elements from the biotite and
chlorite-rich fraction
5.5. Comparison of the release rate of the major elements from
the bulk granite to the dissolution rates of its constituent minerals
5.6. Implications of the present study to the comparison between
dissolution rate in the laboratory and in the field
6. Summary and conclusions
Acknowledgments
References