American Mineralogist, Volume 94, pages 1739–1742, 2009

Letter

Si-Al distribution in high-pressure CaAl4Si2O11 phase: A 29Si and 27Al NMR study
Xianyu Xue,1,* Shuangmeng Zhai,2 and Masami Kanzaki1
1Institute for Study of the Earth’s Interior, Okayama University, Misasa, Tottori, 682-0193 Japan
2
School of Earth and Space Sciences, Peking University, Beijing, 100871, P.R. China

Abstract
High-resolution Si and Al NMR techniques have been applied to resolve the Si-Al distribution
and coordination in the high-pressure CaAl4Si2O11 (CAS) phase, a potentially important mineral in
subducted crustal materials in the deep mantle that has a unique hexagonal ferrite structure containing
two octahedral (M1; M2) and one trigonal bipyramidal sites. The 29Si MAS NMR spectra of the CAS
phase synthesized at 20 GPa and 1400~1600 °C show two broad, asymmetric peaks near –92.7 and–182.7 ppm with an intensity ratio of 1:3, suggesting that 1/4 of the Si are in tetrahedral coordinationand 3/4 in octahedral coordination. Therefore, the trigonal bipyramidal and M1 octahedral sites are each
occupied by equal proportions of Si and Al, and the former are effectively half-occupied face-sharing
tetrahedra (at least for Si). The 27Al MAS and 3Q MAS NMR spectra contain only one unresolved
peak typical of octahedral Al with a range of quadrupolar coupling constants.
Keywords: NMR spectroscopy, Al and Si coordination, CAS phase, Si-Al disorder, high pressure
2927Introduction
The CAS phase is a calcium aluminosilicate that has been
observed in experimental products produced at 14~27 GPa and1500~2400 °C from starting materials of “continental crust composition” (Irifune et al. 1994), the CaO-Al2O3-SiO2 system (e.g.,Gautron et al. 1996), and basaltic compositions (e.g., Hirose andFei 2002). Naturally occurring CAS phase has also been identified
in shocked martian meteorites (Beck et al. 2004). Therefore, thisphase could be an important mineral in crustal materials subductedto the deep mantle. The crystal structure of the CAS phase with aCaAl4Si2O11 composition has been determined by Gautron et al.
(1999) using single-crystal X-ray diffraction (XRD). It belongs
to the family of R-type hexagonal ferrite structure (space groupP63/mmc) that includes several other compounds, such as barium
ferrite (BaFe4Ti2O11). The structure is built with two types ofalternating sheets that are linked by shared corners: one is spinellike sheet containing edge-sharing M1 octahedra, and the otherconsists of pairs of face-sharing M2 octahedra, CaO12 polyhedra,and trigonal bipyramids (Fig. 11). The trigonal bipyramidalsite is in particular unique and unknown to any other silicates:
cations are displaced from the center and seem to reside in two
half-occupied face-sharing tetrahedral (T) sites, like most otherisostructural compounds (Gautron et al. 1999). Because of thesimilarity in X-ray scattering factors of Si and Al, site occupancies
have been inferred indirectly from bond lengths: M1 octahedra
were considered to contain mixed Si and Al, and M2 octahedra
contain only Al. The T sites were thought to be more consistent
with Al occupancy, although Si occupancy could not be excluded.
The fraction and distribution of Si and Al in the M1 sites wereunclear. Such structural information is, however, indispensable
for understanding its physical and thermodynamic properties* E-mail: xianyu@misasa.okayama-u.ac.jp0003-004X/09/1112–1739$05.00/DOI: 10.2138/am.2009.3348(e.g., compressibility, configurational entropy).
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Si and 27Al nuclear magnetic resonance (NMR) spectroscopic techniques are ideal for distinguishing and quantifying
Si and Al of different coordination numbers and/or next-nearest
neighbors (NNN), because the latter often yield peaks of distinct
chemical shift ranges with intensities proportional to site abundances (Kirkpatrick 1988; Xue et al. 2006, 2008). The purposeof this study was to determine the occupancy and coordinationof Si and Al among the different sites in the CaAl4Si2O11 phaseusing high-resolution 29Si magic angle spinning (MAS) NMRand 27Al MAS and triple-quantum (3Q) MAS NMR.

Experimental methods and sample descriptions
Three samples were synthesized from anhydrous starting materials with nominal bulk compositions of CaAl4Si2O11 (samples 885 and 1472) and CaAl2.729Si2O9(sample 1420), respectively. For sample 1420 (29Si-enriched), the starting material
was a mixture of high-purity CaO, Ca(OH)2, Al2O3, and 29Si-enriched SiO2 (95.65%,from ISOTEC, Inc.) that was first loaded into a Pt capsule with one end pre-welded,and then fired at 1000 °C in the capsule for 2 h before welding shut the other end.
All the oxides were fired at 1000~1200 °C for 6~12 h before use. The starting
material for samples 885 and 1472 (natural isotopic abundance) was a mixture ofhigh-purity Al(OH)3, CaCO3, and SiO2 that was dried at 1000 °C for 48 h and thenground in an agate mortar as described in Zhai and Ito (2008).
High-pressure samples were synthesized at 20 GPa and 1400 °C (sample1472) or 1600 °C (samples 1420 and 885) for 8~10 h with a 5000 ton Kawai-type
double-stage uniaxial split-sphere multi-anvil apparatus (USSA-5000) at ISEI. The
high-pressure assembly for sample 1420 was similar to that described in Xue et al.
(2008). A Cr-doped MgO octahedron of 14 mm edge length was used as the pressure
medium, and was compressed with eight WC anvils of 32 mm edge length and 6mm truncation edge length. A stepped LaCrO3 cylindrical heater was embeddedin a ZrO2 thermal isolating sleeve. The sealed sample capsule (2.5 mm O.D., ca. 3
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