Crystal structure of calcium-ferrite type NaAlSiO4 up to 45 GPa.

Alkali-rich aluminous high-pressure phases including calcium-ferrite (CF) type NaAlSiO₄ are thought to constitute ~20% by volume of subducted mid-ocean ridge basalt (MORB) under lower mantle conditions. As a potentially significant host for incompatible elements in the deep mantle, knowledge of the crystal structure and physical properties of CF-type phases is therefore important to understanding the crystal chemistry of alkali storage and recycling in the Earth's mantle. We determined the evolution of the crystal structure of pure CF-NaAlSiO₄ and Fe-bearing CF-NaAlSiO₄ at pressures up to ~45 GPa using synchrotron-based, single-crystal X-ray diffraction. Using the high-pressure lattice parameters, we also determined a third-order Birch-Murnaghan equation of state, with V₀ = 241.6(1) ų, K_T0 = 220(4) GPa, and K T 0 ′ = 2.6 (3) for Fe-free CF, and V₀ = 244.2(2) ų, K_T0 = 211(6) GPa, and ′′ K T 0 ′ = 2.6(3) for Fe-bearing CF. The addition of Fe into CF-NaAlSiO₄ resulted in a 10 ± 5% decrease in the stifest direction of linear compressibility along the c-axis, leading to stronger elastic anisotropy compared with the Fe-free CF phase. The NaO₈ polyhedra volume is 2.6 times larger and about 60% more compressible than the octahedral (Al,Si)O₆ sites, with K 0 N a O 8 = 127 G P a and K 0 (A l , S i) O 6 ∼ 304 G P a. Raman spectra of the pure CF-type NaAlSiO₄ sample shows that the pressure coefficient of the mean vibrational mode, 1.60(7) cm^–1/GPa, is slightly higher than 1.36(6) cm⁻¹/GPa obtained for the Fe-bearing CF-NaAlSiO₄ sample. The ability of CF-type phases to contain incompatible elements such as Na beyond the stability field of jadeite requires larger and less-compressible NaO₈ polyhedra. Detailed high-pressure crystallographic information for the CF phases provides knowledge on how large alkali metals are hosted in alumina framework structures with stability well into the lowermost mantle. [ABSTRACT FROM AUTHOR]