Abstract

The crystal structure of pargasite from a cut gem from Myanmar, (K0.094Na0.93)(Na0.15Ca1.78)(Mg4.51Fe2+0.02Al0.53Ti0.10)(Si6.56Al1.44)O22(OH1.32F0.68), a 9.882(6), b 17.973(11), c 5.282(3) Å, β 105.20(1)°, V 905.33(17) Å3, space group C2/m, Z = 2, has been refined to an R1 index of 1.9% using MoKα single-crystal X-ray diffraction. The unit formula (calculated from the results of electron-microprobe analysis), the refined site-scattering values, and the observed mean bond-lengths, were used to assign site populations. [4]Al occurs at both the T(1) and T(2) sites, but is strongly ordered at T(1). [6]Al is disordered over the M(2) and M(3) sites, but is excluded from the M(1) site. ANa is split between the A(2) and A(m) sites with minor K assigned to the A(m) site. The frequencies of short-range ion arrangements over the configuration symbol M(1)M(1)M(3)–O(3)–A:T(1)T(1) were calculated from the refined site-populations and are in reasonable accord with the fitted infrared spectrum of this amphibole in the principal OH-stretching region.

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