▪ Day, M.C., Hawthorne, F.C., Susta, U., Della Ventura, G., Harlow, G.E. (2018): Gem amphiboles from Mogok, Myanmar: crystal-structure refinement, infrared spectroscopy and short-range order-disorder in gem pargasite and fluoro-pargasite. Mineralogical Magazine, 82, (in press).
The crystal structures of six gem-quality pargasites and fluoro-pargasites from Mogok, Myanmar, space group C2/m, Z = 2, have been refined to R1 indices of 2.20–2.78 % using MoKα 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. TAl occurs at both the T(1) and T(2) sites but is strongly ordered at T(1). Al is partly disordered over the M(2) and M(3) sites but does not occur at the M(1) site. ANa 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.