Gem amphiboles from Mogok, Myanmar: crystal-structure refinement, infrared spectroscopy and short-range order-disorder in gem pargasite and fluoro-pargasite

摘要

The crystal structures of six gem-quality pargasites and fluoro-pargasites from Mogok, Myanmar, space group C2/m, Z = 2, have been refined to R-1 indices of 2.20-2.90% using MoK alpha X-radiation. The unit formulae were calculated from the results of electron-microprobe analysis, and were used with the refined site-scattering values and the observed mean bond lengths to assign site populations. Al-T occurs at both the T(1) and T(2) sites but is strongly ordered at T(1). Al-[6] is partly disordered over the M(2) and M(3) sites but does not occur at the M(1) site. Na-A is split between the A(2) and A(m) sites and K occurs at the A(m) site. The infrared spectra in the principal OH-stretching region were measured and the fine structure was fit to component bands. The component bands were assigned to short-range ion arrangements over the configuration symbol M(1)M(1)M(3)-O(3)-A-O(3):T(1)T(1) using the refined site-populations and the expected frequencies from previously assigned spectra in more simple amphibole compositions, and correspond to the local arrangements: (1) MgMgMg-OH-Na-OH:SiAl; (2) MgMgMg-OH-Na-F:SiAl; (3) MgMgAl-OH-Na-OH:SiAl and (4) MgMgAl-OH-Na-F:SiAl.