Experimental kinetic studies and related theory for solid-state transformations relevant to metamorphic processes unfortunately lag far behind kinetic investigations of mineral-fluid reactions, related to natural waters and their precipitates, and mineral-melt reactions, related to crystallization of igneous rocks (see volume edited by Lasaga and Kirkpatrick, 1981). While metamorphic rocks have experienced successive temperature (T) and pressure (P) changes during their evolution, only rarely can we constrain the magnitude and direction of these P-T changes as functions of time (t). It is at present difficult to say whether characteristic disequilibrium features, observed petrographically, mean that a certain temperature was never achieved, or was only achieved for a short time. Such features might include the coexistence of two polymorphs or any complete reactant plus product assemblage as well as thermodynamically incompatible phases, irregular grain boundaries, and oriented fabrics. A great variety of environmental factors (presence of fluid, deformation, local chemical catalysts, etc.) can influence the kinetics of most mineral transformations. However, it is not often possible to see to what degree such factors have influenced an observed mineral assemblage from petrographic data alone.

Kinetics of Metamorphic Reactions at Elevated Temperatures and Pressures: An Appraisal of Available Experimental Data

The synthesis of titanium oxycarbide by carbothermal reduction of titania was studied in hydrogen, argon, and helium in isothermal and temperature programmed reduction experiments in a tube reactor with continuously flowing gas. In the temperature range of 1000 °C to 1500 °C, the reduction rate increased with increasing temperature. Formation of titanium oxycarbide started at 1200 °C in all three gases. The reduction was the fastest in hydrogen. Formation of titanium oxycarbide in hydrogen was close to completion in 120 minutes at 1300 °C, 60 minutes at 1400 °C, and less than 30 minutes at 1500 °C. The reduction in argon and helium had similar rates and reached 90 to 95 pct after a 300-minute reduction at 1400 °C to 1500 °C. Faster carbothermal reduction of titania in hydrogen than in argon and helium was attributed to involvement of hydrogen in the reaction. Hydrogen reduced titania to titanium suboxides and reacted with carbon, forming methane, which reduced titanium suboxides to titanium oxycarbide. Titanium oxycarbide synthesized in hydrogen for 180 minutes at 1300 °C contained 13 mol pct TiO. At 1500 °C, oxygen concentration decreased to a degree corresponding to 1.4 mol pct TiO. In the titanium oxycarbide produced by a 300-minute reduction at 1600 °C, the TiO content was 0.6 mol pct.

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Carbothermal Reduction of Titania in Different Gas Atmospheres

Stishovite, a form of SiO2 with Si in sixfold coordination, inverts under certain conditions to silica glass with Si in fourfold coordination. The inversion rate determined at ten temperatures between 300° and 800°C for Meteor Crater Stishovite can be extrapolated to the time for total inversion of Stishovite to silica glass at each temperature. The resulting time-temperature curve constitutes a limit for thermal distribution models of Meteor Crater immediately after passage of the shock wave generated by meteoritic impact and initial cooling by adiabatic expansion. The data indicate the virtual impossibility that Stishovite can be formed and preserved at the surface of the earth by any mechanism other than meteoritic impact. The transformation proceeds by nucleation of glass centers along grain boundaries and dislocation surfaces in the Stishovite grains, then by growth from these centers into the host material. The growth rate of the already formed glass centers is the principal mechanism controlling the observed transformation rate in the temperature range of investigation. The transformation appears to proceed by the formation of a dense sixfold-coordinated glass from the Stishovite, then by a breakdown of the dense glass to the observed fourfold-coordinated silica glass.

Observations on the inversion of Stishovite to silica glass

The conversion of boehmite to transition aluminas and to alpha (corundum) was studied with (a)fibrillar colloidal boehmite, about 50 a.u. in diameter, (b)finely divided boehmite in the form of thin elongated platelets, and (c)macroscopic boehmite crystals. In the range 500o to 700o C., (a)was converted to fibrillar gamma, essentially unchanged in size or shape, (b)was dehydrated to gamma, but the platelets appeared to be partly cracked longitudinally about every 70 a.u., and (c)was similarly cracked into much smaller crystallites of gamma. At about 1000oC., (a)yielded only rodlike theta alumina, whereas (b)and (c)appeared to give a mixture of theta and delta aluminas, characterized by round holes or pores developed from the cracks. In the range 1000o to 1100oC., all the aluminas were converted to alpha, at a rate which was strongly dependent on temperature. The conversion of (a)and (b) to alpha could be followed by subsequent examination with the electron microscope. The alpha crystals, once nucleated, apparently grew rapidly to 0.5 or 1.0 μ in size, since there was no evidence of smaller alpha crystals. The smaller alpha crystals appeared to grow while embedded in a glassy matrix which must have been amorphous alumina. This phase acted as a bridge between the diminishing theta particles and the growing alpha crystals and may have consisted of the alumina which was in the process of being transported.

Fibrillar Colloidal Boehmite; Progressive Conversion to Gamma, Theta, and Alpha Aluminas

Very marked differences in the metastable persistence of coesite and stishovite have been demonstrated; the former possibly persists indefinitely below 1000°C at 1 atmosphere "dry," and the latter completely decomposes in minutes above 500° to 600°C to an amorphous or short range order phase. Quartz was grown (metastably) at temperatures well above its stability field from both coesite and stishovite, possibly by way of a short range order phase. The absence of stishovite in meteor-impact craters cannot be taken as evidence that it was not formed. If it has "reversed" in normal natural environments the product would almost certainly be a short range order phase or derivative.

Coesite and Stishovite: Stepwise Reversal Transformations

The transformation of quartz to cristobalite was studied in the temperature range 1400° to 1650° C. The process which was believed to be a direct one was found to be a consecutive reaction involving an intermediate transition phase. Both Madagascar and Brazilian quartz were used to study the transformation reaction. The reaction rate was found to be much faster for the Madagascar quartz than for the Brazilian quartz, although no difference in their impurity content was found in the spectrographic analysis. An approximate consecutive reaction kinetics was applied and the deviation of the experimentally observed values of the transition phase at a particular temperature from that of the mathematical prediction was noted and is discussed.

A. L. Roberts

Papers

Transformation of Quartz to Cristobalite