Al-Mg disorder in a gem-quality pargasite from Baffin Island, Nanavut, Canada

The crystal structure of gem-quality pargasite from Soper River, near Kimmirut, Baffin Island, Nunavut, Canada, (K0.24Na0.73)(Sigma0.97) (Ca1.86Na0.14)(Sigma2.00) (Mg4.15Fe0.07Mn0.01Al0.71Ti0.06)(Sigma5.00) (Si6.45Al1.55) O-22 [(OH)(1.25)F0.63O0.122-](Sigma2.00), a 9.8814(6), b 17.967(1), c 5.2927(4) Angstrom, beta 105.263(5)degrees, V 906,5 Angstrom(3), C2/m, Z = 2, has been refined to an R index of 2.9% using 1506 observed intensities measured with MoKalpha X-radiation. The crystal used for collection of the X-ray intensity data was then analyzed with an electron microprobe. The amphibole composition is very close to that of end-member pargasite. There is significant disorder of Al over the M(2) [1.40 Mg + 0.54 Al + 0.06 Fe3+] and M(3) [0.81 Mg + 0.17 Al + 0.02 Fe2+] sites. Such disorder has been observed in Mg-rich pargasite from a high-pressure paragenesis in the western Alps. The infrared spectrum of this amphibole in the principal OH-stretching region shows a complex envelope that can be resolved into six bands by analogy with the spectra of synthetic (OH,F)-bearing pargasite in the literature. The relative intensities of the component bands are in accord with the observed OH:F ratio in this pargasite, and indicate complete short-range disorder of OH and F. The occurrence of Al-Mg disorder over M(2) and M(3) in the Soper River pargasite, from a lower-pressure environment, indicates that this disorder is a compositional feature of pargasite, rather than a result of crystallization or equilibration at high pressure and temperature. The pattern of electron density in the A cavity of the structure may be interpreted in terms of K and Na at the A(m) site and Na at the A(2) site. Moreover, the specific site-populations are conformable with the composition of this amphibole and the most favorable patterns of short-range order.