『Abstract
Directional dissolution along the three crystallographic axes
of gem-quality olivine (Fo91), San Carlos,
Arizona was studied at pH 1 and pH 2 at 23, 50, 70, and 90℃ and
1 atm. The rate constant of dissolution for olivine at pH 1 and
70℃ down the a-, b-, and c-axis (based on
space group Pbnm) is 2.7×10-4, 5.6×10-3,
and 8.1×10-4mm/h, respectively. At pH 2 and 70℃ the
dissolution rates are 1.3×10-4, 2.1×10-3,
and 4.3×10-4mm/h, respectively. At 50℃ and 90℃, these
rates are 〜0.2 and 5 times the rates at 70℃. 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 Eadis
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 Eadis, dissolution
in the direction down the a-axis will become dominant at
temperatures above 〜140℃. The bulk Eadis,
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 Eadis perpendicular
to the a-axis, bulk Eadis
must increase with temperature. The results indicate that the
weathering rate of olivine is more temperature dependent than
was considered previously.』
1. Introduction
2. Experimental method
2.1. Sample preparation
3. Results and discussion
3.1. Etch-pit formation and evolution
3.2. Dissolution rates
3.3. Activation energy of dissolution
3.4. Olivine structure and dissolution
3.5. Geological applications
Acknowledgments
References