Post-tilleyite, a dense calcium silicate-carbonate phase

Abstract

Calcium carbonate is a relevant constituent of the Earth’s crust that is transferred into the deep Earth through the subduction process. Its chemical interaction with calcium-rich silicates at high temperatures give rise to the formation of mixed silicate-carbonate minerals, but the structural behavior of these phases under compression is not known. Here we report the existence of a dense polymorph of Ca 5 (Si 2 O 7 )(CO 3 ) 2 tilleyite above 8 GPa. We have structurally characterized the two phases at high pressures and temperatures, determined their equations of state and analyzed the evolution of the polyhedral units under compression. This has been possible thanks to the agreement between our powder and single-crystal XRD experiments, Raman spectroscopy measurements and ab-initio simulations. The presence of multiple cation sites, with variable volume and coordination number (6–9) and different polyhedral compressibilities, together with the observation of significant amounts of alumina in compositions of some natural tilleyite assemblages, suggests that post-tilleyite structure has the potential to accommodate cations with different sizes and valencies.

Phase stability and dense polymorph of the BaCa(CO3)2 barytocalcite carbonate

“…The synthetic phase 14 and the HT disordered calcite phase 15 present each Ba/Ca atom octahedrally coordinated. Note that Ca coordination environment in carbonates was previously reported to be related with chemical composition 18 – 21 and to change upon compression 6 , 7 , 22 , 23 .…”

Oxidation of High Yield Strength Metals Tungsten and Rhenium in High-Pressure High-Temperature Experiments of Carbon Dioxide and Carbonates

“…Even though the hotspot was centered, the heat was conducted to the gasket (W)-sample (silicate-carbonate) boundary and a chemical reaction occurred. Here, we report the results of tilleyite compressed to 9 GPa, which already converted into post-tilleyite [6], and laser-heated above 1500 • C.…”

“…High-pressure high-temperature X-ray diffraction (XRD) measurements were performed using symmetric and Boehler-Almax diamond-anvil cells (DACs). The chemical interaction between tungsten and carbonates was observed while laser-heating natural silicate-carbonate samples [6,22,23]. Natural tilleyite mineral (specimen YPM MIN 041104, Yale Peabody Museum) from the Crestmore quarry was analyzed by energy-dispersive X-ray spectroscopy, and only traces (<1 wt%) of Al and K were found apart the Ca, Si, C and O atoms present in the ideal Ca 5 (Si 2 O 7 )(CO 3 ) 2 formulae.…”

“…A primary source for acquiring knowledge of the Earth's interior is, for instance, generating similar pressure and temperature conditions in the laboratory, and studying how these conditions affect metals [1] or minerals [2,3]. A particular interesting problem is to unravel the fate of carbon in the deep mantle [2,[4][5][6], for which HP-HT experiments involving carbon dioxide and carbonates need to be performed. However, the HP-HT chemistry of these compounds is largely unknown, which directly affects the proper design of such experiments.…”

Crystal Structure of BaCa(CO3)2 Alstonite Carbonate and Its Phase Stability upon Compression

“…Thus, whereas CaM(CO 3 ) 2 (M = Mg, Mn, Zn, Fe) present Ca-centered octahedra, − the different Ca–Mg stoichiometry of huntite CaMg 3 (CO 3 ) 4 entails the formation of Ca-centered trigonal prisms . In calcium carbonates that include Na + , K + , Sr 2+ , Ba 2+ cations, Ca coordination varies from 6 to 9 depending on the structure or even within the same structure. − Naturally occurring calcium silicate-carbonates, for instance, present a large variety of Ca atom environments, ranging from 6-fold to 9-fold coordination. , Note that the different classes of irregular cation-centered oxygen polyhedra exhibit a wide range of volumes and bulk moduli, which suggests that other divalent cation species could be accommodated in these sites. Therefore, the study of the atomic arrangements in different Ca carbonate systems and their behavior at high pressures could provide insight into the nature of the Ca–(CO 3 ) interactions and potential chemical substitution at inner earth conditions.…”