Single crystal X-ray and electron microprobe study of Al/Si-disordered anorthite with low content of albite

Abstract

Abstract The crystal chemistry of anorthite with low content of albite (An92.0 Ab3.4), part of a rapidly cooled anorthite megacryst occurring in 1940 ejecta from Miyake-jima volcano, Japan, has been investigated using a single-crystal X-ray diffractometer and an electron microprobe analyzer with wavelength dispersive X-ray spectroscopy (EMPA-WDS). The structure was refined in space group P1̅ and cell parameters, a=8.182(6) Å, b=12.883(4) Å, c=7.092(4) Å, α=93.19(4)°, β=115.91°(4), γ=91.18°(4). The final weighted R-factor is 3.77% for 1549 reflections. Averaged T-O distances are 1.681 Å for T1(0), 1.674 Å for T1(m), 1.677 Å for T2(0) and 1.680 Å for T2(m), indicating Al occupancies of 0.501, 0.453, 0.472, and 0.496, respectively. These results suggest that the Al/Si-distribution in the tetrahedral framework is highly disordered (QOD=0.06), which results in bisection of the c-period in Al/Si ordered anorthites (c∼14 Å). Chemical composition of the refined crystal obtained by EMPA-WDS is (Ca0.93 Na0.03 Fe0.02□0.01)(Mg0.01 Al1.94 Si2.05)O8. The extra-framework site-populations consist of the following two: (1) A(000) site occupied by Ca (86%), Na (8%), Fe2+ (4%) and □(2%), and (2) A(zi0) site by Ca (100%). The Al/Si tetrahedral framework is hence pseudo-face-centered in symmetry (C1ʩ so that both extra-framework cations and their defects appear to reduce the symmetry of the overall structure to P1ʮ Although the Al/Si disordered anorthite can be interpreted as a metastable phase, the observed chemical non-stoichiometry may stabilize such a metastable structure by introducing minor -Si4+-O-Si4+- bonds into the tetrahedral framework and anti-phase-boundary.