Ca-walstromite and its relevance in understanding ring carbonates at deep mantle conditions

CaSiO3 walstromite is the triclinic ring-silicate polymorph of CaSiO3 and is stable between 3 and 9 GPa at ambient temperature. In the studies of the Earth’s interior, it is considered important for two main reasons. Firstly, it is one of the main Ca-bearing phases in the Earth’s mantle transition zone [1]. This has been proved by both laboratory experiments at high pressure (P) and temperature (T) and by natural findings in diamonds [1, 2]. Despite this, its thermoelastic behaviour and its stability at high T are still poorly known. The only existing phase diagram has been determined through multi-anvil experiments, which shows that walstromite transforms to an assemblage of CaSi2O5 + Ca2SiO4 [3]. Secondly, the different CaSiO3 polymorphs constitute a low-P analogue for the study of ultra-high-P carbonate structures at conditions of the lower mantle. Carbonate minerals are considered as one of the main carbon-host phases in the mantle. They are stable from crust to lower mantle conditions, as demonstrated by direct observation of carbonates inclusions in superdeep diamonds [e.g. 4]. To date, the different possible structures adopted by carbonates during their polymorphic phase transitions are still unclear [5], even if recent important experimental [e.g. 6, 7] and theoretical studies [e.g. 8, 9] have demonstrated the transition in complex tetrahedral ring or chain carbonate structures. However, ring-carbonates are unquenchable at ambient P and T conditions and is extremely difficult to perform extreme high-pressure single crystal diffraction with suitable quality of data for accurate structural details determination. Since CaSiO3 polymorphs show the same ring-structure and are quenchable, they constitute evaluable analogue to understand the crystal chemistry of HPcarbonate structures. We performed in-situ high-pressure Diamond Anvil Cell experiments on a synthetic sample of the triclinic Ca-walstromite polymorph at the beamline ID15b at ESRF (Grenoble). The sample was synthesized at 6.5 GPa and 1500°C with a multi-anvil module. We report its phase transition towards a monoclinic structure walstromite-II, at 8.5 GPa, studied by single crystal X-ray diffraction. The monoclinic structure is topologically similar to the triclinic one, but the arrangement of 3-fold ring silicate groups determines a denser structure by about 3%. The walstromite-II structure is significantly denser if compared to wollastonite chain silicate structure. Since the Ca-walstromite is a low-P quenchable structural analogue to 3-fold ring high-P carbonates (Fig. 1), from this preliminar high-P experiment we can envisage that also higher P carbonates, like dolomite IV (Fig. 1), might have a phase transition to higher density structures at extremely high P.1.