Abstract
The crystal structure of pargasite from a cut gem from Myanmar, (K 0.094 Na 0.93 )(Na 0.15 Ca 1.78 )(Mg 4.51 Fe 2+ 0.02 Al 0.53 Ti 0.10 )(Si 6.56 Al 1.44 )O 22 (OH 1.32 F 0.68 ), a 9.882(6), b 17.973(11), c 5.282(3) A, β 105.20(1)°, V 905.33(17) A 3 , space group C2/m , Z = 2, has been refined to an R 1 index of 1.9% using Mo K α 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. A Na 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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