Dear Małgorzata Lempart-Drozd ,

The first heat has a bit lower temperature for ion migration inside the lattice.

However, the second 1050 oC has enough temperature for some ion migration and lattice breakdown, promoting Fe up to the particles' surface.

Since on the surface, the temperature is higher enough to promote reduction. The reduction may happen due to decomposition products of the original sample, carbon from the released gases, impurities from the N2 atmosphere, lack of good vacuum or N2 saturation.. and so on. The reduction may occur by released gases from the original sample and form magnetite.

As higher the temperature rises, the higher the stabilization of formed Fe2+, by reducing free oxygen in the environment. Since in the presence of Fe3+, initially present in your sample, it is natural to have some Fe3O4, as already probed by Mössbauer, a specific Fe probe technic, if Fe57 is used as a radioactive source.

I don't believe the iron silicate crystallizes in magnetite. The oxygen needed to form the magnetic iron oxide comes from the iron silicate decomposition or any other oxygen-containing phases already present at the start after decomposition by the high temperature.

Best regards,

WNM

Dear Wagner Da Nova Mussel,

Thank you very much for your answer.

I'll try to explain my experiments more clearly. I assumed no oxidizing or reducing external agents in my system. Minor Fe2+ oxidation of my starting material occurred in N2 due to oxidative dehydrogenation, (Fe2+ octahedrally coordinated with OH groups is oxidized when H proton release during heating). After thermal decomposition that involves dehydroxylation and dehydrogenation of starting material, ferrosilite is forming (XRD and Mossbauer) at 800deg C – Mossbauer parameters showed that it can be ferrosilite with ~10% of Fe3+ in M1 position. But at 1050 degC I observe ferrosilite and 30% of magnetite, hence the question - Could ferrosilite partially transform into magnetite? The oxygen for magnetite could come from pyroxene… as you claimed ? If it could make difference the starting sample is contaminated by ~20% of quartz.

Dear Małgorzata Lempart-Drozd ,

No, the temperature is too low, even for pyroxene melting, so it is not possible to promote decomposition.

After a long annealing time at 1280 °C, smaller precipitates are in general pure silica.

The partial melting phenomenon occurs for annealings under confining pressure up to 1.6 GPa at T equals 1300 °C.

Their composition became progressively enriched in sodium, aluminum, calcium, and iron oxides, mainly depending on the initial matrix composition.

Naturally annealed pyroxenes (clino- and ortho-) from xenoliths brought to the surface by volcanic explosions also exhibit similar precipitates enriched in silica.

Good luck

WNM

Thank you very much for the answer. Today, I obtained XRD results of sample heated up to 1050degC, besides ferrosilite and magnetite (by Mossbauer) clearly visible fayalite reflections appeared, that only indicates some transformation of ferrosilite.