New insights in the mechanisms of the reaction 3.65 Å phase  =  clinoenstatite + water down to nanoscales

Abstract

The reaction of 3.65 Å phase <=> clinoenstatite + water was investigated in five experiments at 10 GPa, 470–600 ∘C, using a rotating multi-anvil press. Under these P/T conditions, clinoenstatite exists in its high-pressure modification, which, however, is not quenchable to ambient conditions but transforms back to low-pressure clinoenstatite. The quenched run products were characterized by electron microprobe analyses (EMPA), powder X-ray diffraction (XRD), Raman spectroscopy and by high-resolution transmission electron microscopy (HRTEM) on focused ion beam (FIB)-cut foils. We bracketed the reaction in the T range 470 to 510 ∘C (at 10 GPa). The hydration of clinoenstatite to the 3.65 Å phase at 470 ∘C was very sluggish and incomplete even after 96 h. Clinoenstatites range in size from less than 1 to up to 50 µm. Usually clinoenstatite has a very small grain size and shows many cracks. In sub-micron-sized broken clinoenstatite, an amorphous phase (0.91Mg:1.04Si, with about 20 wt % H2O) was observed, which further transformed with increasing reaction time into the 3.65 Å phase (1Mg:1Si, with 34 wt % H2O). Thus, the sub-micron-sized fractured clinoenstatite transformed via an amorphous water-bearing precursor phase to the 3.65 Å phase. The dehydration to clinoenstatite was faster but still incomplete after 72 h at 600 ∘C. From the backscattered electron images of the recovered sample of the dehydration experiment, it is obvious that there is a high porosity due to dehydration of the 3.65 Å phase. Again, the grain size of clinoenstatite ranges from less than 1 up to 50 µm. There are still some clinoenstatite crystals from the starting material present, which can clearly be distinguished from newly formed sub-micron-sized clinoenstatite. Additionally, we observe a water-rich crystalline phase, which does not represent the 3.65 Å phase. Its Raman spectra show the double peaks around 700 and 1000 cm−1 characteristic for enstatite and strong water bands at 3700 and 3680 cm−1. The Mg:Si ratio of 0.90:1.04 was determined by EMPA, totalling to 81 wt %, in accordance with its high water content. Diffraction patterns from high-resolution images (fast Fourier transform – FFT) are in agreement with an orthoenstatite crystal structure (Pbca). The surprising observation of this study is that, in both directions of the investigated simple reaction, additional metastable phases occur which are amorphous in the hydration and crystalline in the dehydration reaction. Both additional phases are water rich and slightly deviate in composition from the stable products 3.65 Å phase and clinoenstatite, respectively. Thus, as a general remark, conventional investigations on reaction progress should be complemented by nanoscale investigations of the experimental products because these might reveal unpredicted findings relevant for the understanding of mantle processes. The extreme reduction in grain size observed in the dehydration experiments due to the formation of nanocrystalline clinoenstatite rather than the slowly released fluids might cause mechanical instabilities in the Earth's mantle and, finally, induce earthquakes.