Description: Multi-anvil and laser-heated diamond anvil methods are used to subject Ge and Si mixtures to pressures and temperatures of between 12 and 17 GPa and 1500 – 1800 K, respectively. Synchrotron angle dispersive X-ray diffraction, precession electron diffraction and chemical analysis using electron microscopy, reveal recovery at ambient pressure of hexagonal Ge-Si solid solutions (P63/mmc). Taken together, the multi-anvil and diamond anvil results reveal that hexagonal solid solutions can be prepared for all Ge-Si compositions. This hexagonal class of solid solutions constitutes a significant expansion of the bulk Ge-Si solid solution family, and is of active interest for optoelectronic applications.

Description: Phengite is known to be an important mineral in the transport of alkalis and water to upper mantle depths. Since ammonium (NH+4) can substitute for K+ in K-bearing minerals, phengite is thus a potential host to transport nitrogen into the mantle. However, the temperature and pressure conditions at which devolatilisation of NH4-bearing phengite occurs are not well constrained. In this study, NH4-phengite (NH4)(Mg0.5Al1.5)(Al0.5Si3.5)O10(OH)2 was synthesised in piston-cylinder experiments at 700°C and 4.0 GPa. Its devolatilisation behaviour was studied by means of in situ micro-FTIR (Fourier transform infrared) spectroscopy under low and high temperatures from -180 up to 600°C at ambient pressure using a Linkam cooling–heating stage and pressures up to 42 GPa at ambient temperature in diamond anvil cell (DAC) experiments. In addition to these short-term in situ experiments, we performed quenched experiments where the samples were annealed for 24 h at certain temperatures and analysed at room conditions by micro-FTIR spectroscopy. Our results can be summarised as follows: (1) an order–disorder process of the NH+4 molecule takes place with temperature variation at ambient pressure; (2) NH+4 is still retained in the phengite structure up to 600°C, and the expansion of the NH+4 molecule with heating is reversible for short-term experiments; (3) kinetic effects partly control the destabilisation of NH+4 in phengite; (4) ammonium loss occurs at temperatures near dehydration; (5) NH+4 in phengite is apparently distorted above 8.6 GPa at ambient temperature; and (6) the local symmetry of the NH+4 molecule is lowered/descended/reduced by increasing pressure (P) or decreasing temperature (T ), and the type and mechanism of this lowered symmetry is different in both cases. The current study confirms the wide stability range of phengite and its volatiles and thus has important implications for the recycling of nitrogen and hydrogen into the deep Earth. Moreover, it is considered as a first step in the crystallographic determination of the orientation of the NH+4 molecule in the phengite structure.

Description: 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 lowpressure 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 cm1 characteristic for enstatite and strong water bands at 3700 and 3680 cm1. 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.

Description: The high-pressure CaCO3 phase diagram has been the most extensively studied within the carbonates group. However, both the diverse mineralogy of carbonates and the abundance of solid solutions in natural samples require the investigation of multi-component systems at high pressures (P) and temperatures (T). Here we studied a member of the CaCO3–SrCO3 solid solution series, and revealed the effect of substituting Ca2+ with Sr2+ on the pressure-induced phase transitions in calcium carbonate. A synthetic solid solution Ca0.82Sr0.18CO3 was studied in situ by Raman spectroscopy in a diamond anvil cell (DAC) up to 55 GPa and 800K. The results of this work show significant differences in the high-pressure structural and vibrational behavior of the (Ca, Sr)CO3 solid solution compared to that of pure CaCO3. The monoclinic CaCO3-II-type structure (Sr calcite-II) was observed already at ambient conditions instead of the ‘expected’ rhombohedral calcite. The stress-induced phase transition to a new high-pressure modification, termed here as Sr-calcite-IIIc, was detected at 7GPa. Sr-calcite-VII formed already at 16GPa and room T, which is 14GPa lower compared to CaCO3-VII. Finally, crystallization of Sr-aragonite was detected at 540K and 9GPa, at 200K lower T than pure aragonite. Our results indicate that substitution of Ca2+ by bigger cations, such as Sr2+, in CaCO3 structures can stabilize phases with larger cation coordination sites (e.g. aragonite, CaCO3-VII, and post-aragonite) at lower P –T conditions compared to pure CaCO3. The present study shows that the role of cationic composition in the phase behavior of carbonates at high pressures should be carefully considered when modeling the deep carbon cycle and mantle processes involving carbonates, such as metasomatism, deep mantle melting,and diamond formation.