Forsteritic olivine: Effect of crystallographic direction on dissolution kinetics

Directional dissolution along the three crystallographic axes of gem-quality olivine (Fo(91)), San Carlos, Arizona was studied at pH 1 and pH 2 at 23, 50, 70, and 90 degrees C and 1 atm. The rate constant of dissolution for olivine at pH 1 and 70 degrees C down the a-, b-, and c-axis (based on space group Pbnm) is 2.7 x 10(-4), 5.6 x 10(-3), and 8.1 x 10(-4) mm/h, respectively. At pH 2 and 70 degrees C the dissolution rates are 1.3 x 10(-4), 2.1 x 10(-3), and 4.3 x 10(-4) mm/h, respectively. At 50 degrees C and 90 degrees C, these rates are similar to 0.2 and 5 times the rates at 70 degrees C. The much higher dissolution rate in the direction of the b-axis is attributed to preferential protonation of the oxygen atoms around the M(1) site, which would result in a higher dissolution rate of the SiO2-M(1) network. The activation energy of dissolution E-a(dis) in the direction down the a-, b-, and c-axis is 114.5 +/- 23 kJ/mol, 69.9 +/- 8 kJ/mol, and 72.9 +/- 15 kJ/mol, respectively. Because of differences in the directional E-a(dis), dissolution in the direction down the a-axis will become dominant at temperatures above similar to 140 degrees C. The bulk E-a(dis), based on the dissolution rate along the crystallographic axes, is 71.5 +/- 12 kJ/mol at the temperature range of the study. Because of the larger E-a(dis) perpendicular to the a-axis, bulk E-a(dis) must increase with temperature. The results indicate that the weathering rate of olivine is more temperature dependent than was considered previously. Copyright (C) 2000 Elsevier Science Ltd.