Deformation and Transformation Textures in the NaMgF3 Perovskite→Post-Perovskite System

Abstract:

MgSiO₃ post-perovskite (pPv) is believed to be a major mineral component in the lowermost mantle. However MgSiO₃ pPv is only stable above ~125 GPa making deformation experiments on this phase particularly challenging. Thus it is of interest to determine suitable analogs for MgSiO₃ pPv. NaMgF₃ is isostructural with MgSiO₃ perovskite (Pv) at ambient conditions and transforms to the pPv structure at ~30 GPa, making this system a potentially useful analog. Here we report on deformation and texture development in the NaMgF₃Pv-pPv system.

During room temperature compression in the diamond anvil cell, NaMgF₃ Pv rapidly develops a 100 texture. Simulations using the visco plastic self-consistent code (VPSC) indicate that a 100 texture in Pv is due to (100) slip or twinning on {110}<1-10>. After inducing the transformation to pPv by laser heating at ~ 30 GPa, NaMgF₃ pPv exhibits a texture maximum near {110} indicating that {100}_Pv → {110}_pPv. This is consistent with transformation mechanisms proposed by theoretical work (Tsuchiya et al 2004; Oganov et al 2005) and with experimental work on MgGeO₃ (Miyagi et al 2011) and NaNiF₃(Dobson et al 2013).

Upon further compression to 66 GPa the 110 textures disperses and develops a maximum toward 001 with a minimum near 100. VPSC modeling was performed using the 110 transformation texture as a starting texture for the simulations. (010)<101> slip generates a strong maximum at 010 and a minimum at 001. Slip on (001)<100> results in a maxima near 110 with a shoulder close to 001, similar to the experimental deformation texture. Thus it is most likely that at room temperature, NaMgF₃ pPv slips predominantly on the (001) plane, consistent with MgSiO₃ pPv (Miyagi et al 2010) and MgGeO₃ (Miyagi et al 2011)_.

Dobson, D. P., et al., Nature Geoscience, 6(7), 575–578 (2013)

Miyagi, L., et al., Science, 329(5999), 1639 –1641 (2010).

Miyagi, L., et al., Physics and Chemistry of Minerals, 38(9), 665–678 (2011)

Oganov, A. R., et al., Nature, 438(7071), 1142–1144 (2005).

Tsuchiya, T., et al., Earth and Planetary Science Letters, 224(3-4), 241–248 (2004)