GEM PARGASITE FROM MYANMAR: CRYSTAL STRUCTURE AND INFRARED SPECTROSCOPY

The crystal structure of pargasite from a cut gem originating from Myanmar, (K_0.094Na_0.93) (Na_0.144Ca_1.781) (Mg_4.513Fe²⁺_0.023Al_0.529Ti_0.10) (Si_6.557Al_1.443) O₂₂ (OH_1.56F_0.44), a = 9.882(6), b = 17.973(11), c = 5.282(3) Å, β = 105.20(1)°, V = 905.33(17) ų, space group C2/m, Z = 2, has been refined to an R₁ index of 3.2 % 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 infrared spectra of amphiboles have information concerning the short-range order of cations and anions in the amphibole structure. This information can rarely be extracted from natural amphiboles as their chemical compositions give rise to many bands in the principal OH-stretching region and the resultant spectra do not contain enough information to fit them correctly; consequently, most of our knowledge of short-range order comes from work on synthetic amphiboles. However, gem-quality minerals (particularly amphiboles) are generally of much simpler composition than common amphiboles particularly because they have very low Fe contents (often because the Fe is tied up in pyrite), and the presence of Mg and Fe increases the number of bands by a factor of four relative to Fe-free amphiboles. The powder infrared spectrum of gem pargasite shows an envelope centered on ~3695 cm^-1 with two prominent maxima at 3710 and 3686 cm^-1 and a slight shoulder at ~3660 cm^-1. The short-range arrangements involving the Mg/Al over the M(2) and M(3) sites, Si/Al at the T(1) site, and OH/F at the O(3) site were identified in the infrared spectrum through chemical constraints and comparison with the spectra of synthetic amphiboles. As with other complicated monoclinic amphiboles, the short-range order observed seems to be dictated by the valence-sum rule of localized bond-valence theory.