KINETIC AND THERMODYNAMIC PROPERTIES OF MOGANITE, A NOVEL SILICA POLYMORPH

A growing body of evidence reveals that much of the silica that crysta llizes at the Earth's surface is a finely intergrown mixture of quartz and moganite. To better understand the behaviour of both solid and aq ueous silica in these systems, the kinetics and thermodynamic properti es for endmember moganite have been determined as a function of temper ature from 25 degrees to 200 degrees C, Because endmember moganite has yet to be found in nature or synthesized in the laboratory, these pro perties were determined indirectly by (1) measuring quartz dissolution rates at pH 3.5, (2) measuring the dissolution rates of quartz/mogani te mixtures of various proportions at pH 3.5 to deduce the endmember m oganite dissolution rate, (3) using the principle of detailed balancin g and the assumption that silica polymorphs have equal precipitation r ates (Rimstidt and Barnes, 1980) to compute the equilibrium constant f or the quartz to moganite transformation reaction, and (4) regressing these data together with corresponding values for quartz to generate e ndmember moganite thermodynamic properties. Equations describing the t emperature dependence of the specific dissolution rate of quartz, k(+, Si,qtz) (mole/m(2)/s), and moganite, k(+,Si,mog) (mole/m(2)/s) at pH 3 .5 and far from equilibrium are [GRAPHICS] which is consistent with ac tivation energy of 80.5 and 70.5 kJ/mol for quartz and moganite, respe ctively. The specific dissolution rate of moganite is 7.4 times faster than that of quartz at pH 3.5 and 25 degrees C. The surface area of q uartz/moganite mixtures increase exponentially with increasing moganit e content. It follows that the apparent dissolution and precipitation rate of quartz/moganite mixtures also increases exponentially with mog anite content. The standard state enthalpy and Gibbs free energy of fo rmation for moganite from the elements at 25 degrees C and one bar was calculated to be -900.723 and -851.314 kJ/mole which is 10 and 5 kJ/m ol, respectively, more positive than those for quartz. The standard st ate entropy at these conditions is 58.245 J/mole/K, which is 17 J/mol/ K greater than that for quartz. The logarithm of the equilibrium const ant for moganite hydrolysis is -3.14 at 25 degrees C and one bar, whic h corresponds to a solubility of 44 mg/kg silica. In contrast, the log arithm of the equilibrium constant for quartz hydrolysis is -4.00 whic h corresponds to a solubility of 6 mg/kg silica. The difference in the hydrolysis constants decreases with increasing temperature. The relat ive rapid dissolution rate of moganite and its thermodynamic instabili ty with respect to quartz is consistent with the observation (Heaney a nd Post, 1992) that moganite is depleted in weathered chert and chalce dony, and it supports the diagenetic silica sequence proposed by Heane y (1995), who documented a scarcity of moganite in rocks older than 10 0 m.y. It also follows that the high abundance of moganite in recent a rid environments is likely due to the lack of water available to media te the dissolution of moganite and simultaneous precipitation of quart z. Copyright (C) 1997 Elsevier Science Ltd.