The effect of volatiles on reaction rim growth dynamics (2019-2023, FWF project P31787)

Metamorphic reaction rims form during the incomplete solid-solid reaction of two neighboring minerals. Parameters that significantly influence the formation of reaction rims are, for example, pressure, temperature, the duration of the reaction or the chemistry of the system. Consequently, naturally occurring reaction rims contain information about their respective formation conditions. However, in order to infer these, we need detailed information about the exact effect of each parameter on the reaction rim growth dynamics.

Numerous scientific studies have already shown that small amounts of volatiles such as H2O can have a significant influence on the formation and growth of reaction rims. In this project, we investigated the effect of very small amounts of water on the formation of reaction rims. However, the addition of extremely small amounts of water (order of magnitude: 0.001 wt%) to high-pressure experiments posed a major experimental challenge, as a dried powder already adsorbs water on the surface of the grains in such a large amount that we need to refer to the system as "wet". Therefore, in order to add extremely small amounts of water in a controlled manner to an experiment, synthetic periclase single crystals experiments that were "doped" with minute amounts of OH-defects were used as starting materials for rim growth. During an experiment, these "doped" crystals release their structurally bound water, which can then affect as “active water” the growth of reaction rims.

With this method, it could be shown experimentally that presence of ppm-amounts of water can be a necessary catalyst that is required to initiate rim growth. Furthermore, an existing model was revised, which now shows how smallest amounts of "active water" effect the growth rate and the thickness ratio of individual layers in a reaction rim. This implies that reaction rims have the potential to be used as geohygrometers, which means that they can provide information about the presence of minute amounts of water that were present long ago during a metamorphic reaction.


Figure 1: Effect of fluorine on the reaction rim thickness, phase sequence and microstructure in two experiments performed at identical P-T-t-conditions.

However, natural fluids contain numerous other volatile elements, such as nitrogen, carbon, fluorine, chlorine, bromine or iodine. All these elements can also have a significant effect on the formation and growth dynamics of metamorphic reaction rims. In the second part of the project, we investigated experimentally the effect of fluorine on the formation and growth of reaction rims (Fig. 1). Results showed that presence of fluorine may strongly increase the growth rate of reaction rims and can have an influence on the phase stability and thus the sequence and texture of the individual layers in a multilayered reaction rim sequence. This means that reaction rims can also provide information about the composition of complex fluids that were present during metamorphism and can consequently be used as geofluidometers.


Franke MG, Joachim-Mrosko B (2022) The effect of fluorine on reaction rim growth dynamics in the ternary CaO-MgO-SiO2 system. American Mineralogist, DOI:

Franke MG, Schmidt B, Stalder R and Joachim-Mrosko B (2023) Metamorphic reaction kinetics at anhydrous to water saturated conditions in the binary MgO-SiO2 system. Contributions to Mineralogy and Petrology 187, 87.

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