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Stability of the MgSiO 3 analog NaMgF 3 and its implication for mantle structure in super‐Earths
Author(s) -
Grocholski B.,
Shim S.H.,
Prakapenka V. B.
Publication year - 2010
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2010gl043645
Subject(s) - mantle (geology) , pyrite , phase transition , convection , phase (matter) , materials science , geology , thermodynamics , mineralogy , chemistry , geophysics , physics , organic chemistry
First‐principles calculations on MgSiO 3 suggested a breakdown into MgO + SiO 2 at pressure above 1000 GPa with an extremely large negative Clapeyron slope, isolating the lowermost mantles of larger super‐Earths (∼10 M ⊕ ) from convection. Similar calculations predicted the same type of breakdown in NaMgF 3 to NaF + MgF 2 at 40 GPa, allowing for experimental examination. We found that NaMgF 3 is stable to at least 70 GPa and 2500 K. In our measurements on MgF 2 (an SiO 2 analog), we found a previously unidentified phase (“phase X”) between the stability fields of pyrite‐type and cotunnite‐type (49–53 GPa and 1500–2500 K). A very small density increase (1%) at the pyrite‐type → phase X transition would extend the stability of NaMgF 3 relative to the breakdown products. Furthermore, because phase X appears to have a cation coordination number intermediate between pyrite‐type (6) and cotunnite‐type (9), entropy change (Δ S ) would be smaller at the breakdown boundary, making the Clapeyron slope ( dP / dT = Δ S /Δ V ) much smaller than the prediction. If similar trend occurs in MgSiO 3 and SiO 2 , the breakdown of MgSiO 3 may occur at higher pressure and have much smaller negative Clapeyron slope than the prediction, allowing for large‐scale convection in the mantles of super‐Earth exoplanets.