Rates and mechanisms of quartz dissolution with geological applications

• Main Author: Gratz, Andrew J
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-1990
• Subjects: Geochemistry; Mineralogy
• ISBN: 9798207235196

The quartz dissolution process was investigated in detail. A highly accurate technique was developed for measuring dissolution rates and the effects of crystal orientation on rate; results include an activation energy of $\sim$87 kJ/mol and rate constants for individual faces. Rates increased with alkalinity and ionic strength and decreased linearly with silica content. Hydroxides of Li, Na, and K dissolved quartz at nearly identical rates. Dissolution surfaces (including etch structures) were studied by optical microscopy, SEM, and APM. Dissolution surfaces consisted of atomic-scale steps with pseudo-regular spacings which varied very little either with temperature or solution chemistry. Thus, the dissolution rate can be factored into a constant ledge density and a temperature-activated ledge velocity. Etch pits were also imaged in AFM. The stability theory of etch pit formation explains many aspects of quartz etching, but a kinetic treatment is required to explain important details. Experiment and theory suggest that the presence or absence of etch pits in natural waters will be sensitive to solution chemistry, but interpretation of these structures requires re-etching of samples. XPS of dissolution surfaces indicated that cations are at best weakly bound to the quartz surface, so that dissolution can be modeled without treating cation-silica interactions when Li, Na, and K are the metals in solution. A ledge-motion model specific to the quartz-water system is derived and shown to be consistent with rates and surface structures. This is turn allows the observed rate constants and activation energies to be associated with elementary atomic processes. Two possible reaction schemes were considered, and it was not possible to decide which (if either) of them is rate-controlling. Results are summarized in the form of a rate law applicable to either facets or powders. Much of the surface area measured by BET is not available for dissolution: only the nominal surface area of a powder normal as determined from grain diameters need be considered. Defects had a small effect on dissolution rate at low densities, but at high densities disrupted the crystal surface. This will tend to affect long-term dissolution behavior. The above results were combined with data on grain boundaries to derive a rate law for solution transfer deformation based on a hierarchical plumbing system.