Structural and chemical characterization of Mg[(Cr,Mg)(Si,Mg)]O (sub 4) , a new post-spinel phase with sixfold-coordinated silicon
Structural and chemical characterization of Mg[(Cr,Mg)(Si,Mg)]O (sub 4) , a new post-spinel phase with sixfold-coordinated silicon
American Mineralogist (July 2015) 100 (7): 1633-1636
- chromium
- coordination
- crystal chemistry
- crystal structure
- electron probe data
- experimental studies
- formula
- geologic barometry
- geologic thermometry
- high pressure
- lattice parameters
- mantle
- metals
- mineral assemblages
- nesosilicates
- olivine group
- orthosilicates
- oxides
- pressure
- ringwoodite
- silicates
- space groups
- spinel group
- spinel structure
- synthesis
- synthetic materials
- wadsleyite
- X-ray diffraction data
- post-spinel structure
The crystal structure and chemical composition of a crystal of Mg(Mg,Cr,Si) (sub 2) O (sub 4) post-spinel phase synthesized in the model system MgCr (sub 2) O (sub 4) -Mg (sub 2) SiO (sub 4) at 16 GPa and 1600 degrees C have been investigated. The compound was found to crystallize with a distorted orthorhombic calcium-titanate (CaTi (sub 2) O (sub 4) ) structure type, space group Cmc2 (sub 1) , with lattice parameters a = 2.8482(1), b = 9.4592(5), c = 9.6353(5) Aa, V = 259.59(1) Aa (super 3) , and Z = 4. The structure was refined to R (sub 1) = 0.018 using 345 independent reflections. The loss of the inversion center is due to the ordering of cations at the octahedral sites: Cr is mainly hosted at the M1 site, whereas Si at the M2 site. Such an ordered distribution induces a distortion thus provoking a change in coordination of Mg, which becomes sevenfold-coordinated instead of the usual eightfold coordination observed in post-spinel phases. Electron microprobe analysis gave the Mg[(Cr (sub 0.792) Mg (sub 0.208) )(Si (sub 0.603) Mg (sub 0.397) )]O (sub 4) stoichiometry for the studied phase. The successful synthesis of this phase can provide new constraints on thermobarometry of wadsleyite/ringwoodite-bearing assemblages.
