Enstatite evaporation kinetics and gas-phase speciation: in-situ experiments constrain protoplanetary disk processes

Enstatite (MgSiO₃) is a major phase in the condensation sequence of the solar nebula. Under equilibrium conditions enstatite forms via the reaction of previously condensed forsterite with a SiO-bearing vapor phase. However, in the case of kinetically limited condensation this reaction is inhibited and the disequilibrium assemblage of forsterite and quartz forms [1]. Groundbreaking observations with the James Webb Space Telescope (JWST) have allowed the direct detections of forsterite and enstatite together with a SiO gas in the protoplanetary disk around the young star HOPS-315 [2]. A quantitative understanding of the kinetics of these condensation reactions is therefore critical for both the formation of the first building blocks of the Solar System planets, and constraining observations of exoplanet formation around young stars. Previously, experiments have documented the incongruent nature of enstatite evaporation in vacuum [3]. Here, for the first time, we present in-situ measurements of enstatite evaporation in vacuum allowing the precise determination of the activation energy and the rate of evaporation, directly coupled with the observation of the evolved gas species.

In the ELMO (Experimental Laboratory Magma Ocean) laboratory we conduct experiments in a simultaneous thermal analyzer (NETZSCH STA 449 F3 Jupiter) under high vacuum conditions and temperatures of up to 2000 °C using a water-cooled graphite furnace. A quadrupole mass spectrometer is directly coupled and in the same vacuum as the sample, allowing in-situ measurement of the evolved gas species. Enstatite single crystals with masses of ~1 mg are heated at different rates from 1 °C/min to 10 °C/min up to 1900 °C. Temperature is monitored with a type W thermocouple providing accurate measurements of ±0.1 °C. Mass change during the evaporation is measured at a resolution of 0.1 µg even under vacuum and very high temperatures. This allows detailed controlled rate thermal analysis of the enstatite evaporation.

The evaporation of enstatite under vacuum conditions is known to be incongruent, producing a forsterite-enriched residue as enstatite preferentially loses SiO and oxygen to the gas phase. This incongruent behavior is of direct relevance to protoplanetary disk processes, as it implies that the reverse reaction – the condensation of enstatite from a forsterite-bearing assemblage and a SiO-rich vapor – is also kinetically controlled, with an activation energy that determines whether enstatite can form on timescales relevant to the cooling solar nebula. However, the precise activation energy of enstatite evaporation, as well as the identity and relative abundances of the evolved gas species under controlled conditions, remain poorly constrained. In particular, it is unclear whether SiO is the dominant gas species released during evaporation, or whether other Si- and O-bearing molecules contribute significantly to the evaporative flux under varying temperature and heating rate conditions.

The controlled-rate thermal analysis approach employed here allows the activation energy of enstatite evaporation to be extracted through kinetic analysis of the mass loss curves obtained at different heating rates, using established isoconversional methods. The simultaneous in-situ measurement of evolved gas species by the coupled quadrupole mass spectrometer directly constrains the gas-phase speciation during evaporation, allowing identification of the dominant vapor-phase carriers of silicon and oxygen. These data will provide the first self-consistent experimental dataset linking evaporation kinetics and gas-phase chemistry, directly applicable as input parameters to kinetic condensation models such as KineCond [1]. Ultimately, the results of this study on enstatite, as well as other oxides and sulfides [4,5] will allow quantitative assessment of whether condensation processes can keep pace with cooling in the solar nebula under realistic disk conditions, and will provide new observational benchmarks for interpreting JWST detections of silicate gas and dust in actively forming planetary systems.

[1] Charnoz, S., Aléon, J., Chaussidon, M., Sossi, P.A., Marrocchi, Y., Franco, P. (2026) Non-equilibrium condensation of the first Solar System solids. Nature, 652:925-930. https://doi.org/10.1038/s41586-026-10257-5.

[2] McClure, M.K., van’t Hoff, M., Francis, L., Bergin, E., Rocha, W.R.M., Sturm, J.A., Harsono, D., van Dishoeck, E.F., Black, J.H., Noble, J.A., Qasim, D., Dartois, E. (2025) Refractory solid condensation detected in an embedded protoplanetary disk. Nature, 643:649-653. https://doi.org/10.1038/s41586-025-09163-z.

[3] Tachibana, S., Tsuchiyama, A., Nagahar, H. (2002) Experimental study of incongruent evaporation kinetics of enstatite in vacuum and in hydrogen gas. Geochimica et Cosmochimica Acta, 66:713-728. https://doi.org/10.1016/S0016-7037(01)00797-9.

[4] Abel, A., Renggli, C.J. (2026) From Magma Oceans to Atmospheres: Copper Volatility as a Tracer of Magma Ocean Outgassing. Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-108. 

[5] Erftemeijer, R.C., Renggli, C.J. (2026) Experimental study on troilite evaporation to constrain condensation processes from the solar nebula. Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-239.