Effects of Mineral Spatial Patterns on Magnesite Dissolution Rates

This data set archives several years’ data of flow-through column experiments exploring the largely unknown effects of mineral spatial distribution on dissolution rates using columns packed with the same magnesite mass but with different distributions within a quartz matrix. Variables include the spatial distribution of the reactive mineral magnesite, orientation of the magnesite zone to the main flow direction, length scale, flow rates, and size contrast between magnesite and quartz grains. The columns with lengths varying from 5 to 22 cm were flushed with acidic solutions (pH 4.0) at flow velocities varying from 0.015 to 7.2 m/d. The largest rate difference was observed between a “Mixed” column containing uniformly distributed magnesite and a “One-zone” column containing magnesite in one cylindrical zone in the center of the column (“flow-parallel One-zone” column). Breakthrough data show that the zonation in the One-zone columns, especially when the grain sizes of magnesite are smaller than that of the quartz, limits magnesite dissolution by a factor of 1.5 – 4.0 compared to the Mixed column. The magnitude of the rate difference increases with increasing flow rates. Under low flow rate conditions, the pore fluids reach chemical equilibrium and reaction kinetics does not play an important role. For those conditions, the mineral distribution does not make a difference. Under high flow conditions, however, the effects of the mineral distribution are maximized because column-scale rates exhibit kinetic control due to low residence times. The rate differences between the Mixed and the flow-parallel One-zone cases are much larger than the 14% maximum difference observed between the Mixed column and the “flow-perpendicular One-zone” column. Thedata set identifies general principles that govern mineral dissolution rates in heterogeneous porous media. It will advance our models of the multi-component geochemical reactions in natural, heterogeneous porous media and will provide valuable insights on the laboratory-field rate discrepancy.

本数据集归档了数年的渗流柱实验数据，旨在探究此前尚未明确的矿物空间分布对溶解速率的影响。实验所用柱体装填的菱镁矿（magnesite）质量一致，但菱镁矿在石英（quartz）基质中的空间分布各不相同。实验变量包括反应性矿物菱镁矿的空间分布、菱镁矿区域与主流方向的取向、特征长度尺度、流体流速，以及菱镁矿与石英颗粒的粒径对比度。
实验柱体长度介于5至22 cm之间，以pH值为4.0的酸性溶液进行渗流冲洗，流速范围为0.015至7.2 m/d。实验中观测到的最大速率差异出现在两类柱体之间：一类为“均匀分布型”（Mixed）柱体，其内菱镁矿呈均匀分布；另一类为“单区域型”（One-zone）柱体，其内菱镁矿集中于柱体中心的单一圆柱形区域，即“顺流单区域型”（flow-parallel One-zone）柱体。
突破曲线数据（breakthrough data）显示，单区域型柱体中的矿物分带结构（尤其是当菱镁矿粒径小于石英时）会抑制菱镁矿溶解，相较于均匀分布型柱体，其溶解速率被降低1.5至4.0倍。速率差异的幅度随流速升高而增大：在低流速条件下，孔隙流体达到化学平衡（chemical equilibrium），反应动力学（reaction kinetics）不再发挥主导作用，此时矿物分布不会对溶解速率产生影响；而在高流速条件下，由于流体停留时间较短，柱体尺度的溶解速率受动力学控制，矿物分布的影响达到最大化。
均匀分布型柱体与顺流单区域型柱体之间的速率差异，远大于均匀分布型柱体与“垂直流单区域型”（flow-perpendicular One-zone）柱体之间观测到的14%最大差异。
本数据集明确了调控非均质多孔介质（heterogeneous porous media）中矿物溶解速率的通用原则，将推动天然非均质多孔介质内多组分地球化学反应（multi-component geochemical reactions）模型的发展，并为实验室与野外速率差异问题提供极具价值的认知参考。