X-ray diffraction and Raman spectroscopy earlier revealed partial decomposition of forsterite (Mg₂SiO₄) to MgO and SiO₂ at hydrogen pressures 2–10 GPa and T~1000 K [1,2]. Recently [3], we observed the hydrogen-induced decomposition of another end-member olivine compound, fayalite (Fe₂SiO₄), at much lower temperatures varying from ~375 °C at P = 1.4 GPa to ~175 °C at P = 7.5 GPa. Fayalite was shown to completely decompose to a mixture of silica, water and metallic Fe or FeH when it was in contact with more than 5 moles of molecular hydrogen (H₂ / Fa ≥ 5).

However, fayalite can be in contact with a smaller amount of hydrogen (H₂ / Fa < 5) in the natural conditions. Our paper reports on the decomposition of fayalite at P = 7.5 GPa and T = 280 °C in the presence of deuterium taken in the molar ratios D₂ / Fa = 5, 2.2 and 1.6. The reaction products were analyzed by quadrupole mass-spectroscopy, so we used deuterated samples to be sure that they could only be formed in the reaction of fayalite with the D₂ gas.

Powder samples of Fe₂SiO₄ were exposed to P_D2 = 7.5 GPa and T =280 °C for 24 hours. Then, the samples were quenched to liquid N₂ temperature, the pressure was released; the high-pressure chamber was disassembled under liquid nitrogen; the samples were retrieved from the reaction cell and stored in liquid N₂ until the measurements.

When heated in the mass spectrometer, each quenched sample released heavy water and deuterium gas from decomposing solid solutions SiO₂-D₂ and FeD compound. The presence of the heavy water witnessed the decomposition of fayalite during its deuteration at high pressure and temperature. The samples deuterated with D₂ / Fa = 2.2 and 1.6 also released gases with m/e = 30 and 32. In the process of heating, the pressures of these gases changed simultaneously, which indicated their origin from one compound. This compound is likely to be silane SiD₄. However, the maximal value of pressures of the gases with m/e = 30 and 32 is smaller by the one or two orders of magnitude compared to those for D₂O and D₂, therefore silane could only be considered as an impurity to heavy water and molecular deuterium.

X-ray diffraction showed that the phase composition of the quenched samples varied with the deuteration pressure. The sample deuterated with the molar ratio D₂ / Fa = 5 consisted of iron deuteride (61 mol.%) and coesite (39 mol.%). The samples deuterated with the molar ratios D₂ / Fa = 2.2 and 1.6 contained no iron deuteride and were mainly composed of hematite Fe₂O₃, magnetite Fe₃O₄, bcc iron and quartz or coesite phases of SiO₂.

The work was partly supported by grant No. 18-02-01175 from the Russian Foundation for Basic Research.

1. Shinozaki A, et al. (2012.) Phys Chem Minerals 39:123–129.
2. Shinozaki A, et al. (2013) Americ Min 98:1604-1609.
3. Efimchenko VS, et al. (2019) Phys Chem Minerals. https://doi.org/10.1007/s00269-019-01035-z