Cation sites in Al-rich MgSiO₃perovskites

Al environment in tectosilicate and peraluminous glasses: A 27Al MQ-MAS NMR, Raman, and XANES investigation

“…In addition, there is a broad feature at ϳ1581 eV. The peak a corresponds to a 1s 3 3p transition (Li et al, 1995) and its energy position has been previously used to quantify the [4] Al/ [6] Al ratio (Li et al, 1995;Andrault et al, 1998;Ildefonse et al, 1994). For glasses with R ϭ CaO/Al 2 O 3 ϭ 1, peak a is located at 1565 eV and can be attributed to Al in four-fold coordination as for other CAS glasses with CaO/Al 2 O 3 Ն 1 .…”

“…At constant SiO 2 content, the position of the resonance a shifts towards higher energy (ϩ0.5 eV), especially for the CA33.43 and CA10.55 glasses. This effect can be attributed to the presence of a small amount (few %) of Al in five or sixfold coordination that exhibit a resonance a at higher energy than for the four fold coordinated cation (Ildefonse et al, 1994;Li et al, 1995;Andrault et al, 1998). In contrast, the position of peaks a is not significantly affected by the addition of Al 2 O 3 in silica-rich glasses (76 and 50 mol% SiO 2 ).…”

Experimental constraints on crystallization differentiation in a deep magma ocean

“…Divalent major element cations (e.g., Ca, Mg, Fe 2ϩ ) reside in the eightfold site, whereas Si 4ϩ resides in the octahedral site. Although there is still debate about the substitution mechanisms, Al 3ϩ may mix into A and B sites via coupled substitution with Mg 2ϩ and Si 4ϩ , or into the B site accompanied by vacancy formation (e.g., Andrault et al, 1998;Richmond and Brodholt, 1998). A coupled substitution of Al 3ϩ and Fe 3ϩ for Si 4ϩ and Mg 2ϩ is also likely, with Fe 3ϩ mixing into the A site and Al 3ϩ into the B site (e.g., Frost and Langenhorst, 2002).…”

Mössbauer and ELNES spectroscopy of (Mg,Fe)(Si,Al)O3 perovskite: a highly oxidised component of the lower mantle

“…Fe 3+ has been inferred to occupy the B sites in (Mg,Fe)SiO 3 perovskite synthesised at minimum oxygen fugacity, but may occupy both A and B sites in samples with higher Fe 3+ /SFe (McCammon 1998). Al 3+ has been inferred to occupy both the A and B sites in Al-rich MgSiO 3 perovskite (Andrault et al 1998).…”