Abstract

The crystal structures of gem-quality richterite and pargasite from Afghanistan, space group C2/m, Z = 2, have been refined to R1 indices of 2.47% and 3.22%, respectively, using MoKα X-radiation. Results from electron-microprobe analysis were used to calculate unit formulae and site populations were assigned using the refined site-scattering values and the observed mean bond-lengths. In pargasite, [4]Al is strongly ordered at T(1) and [6]Al is partly disordered over the M(2) and M(3) sites, whereas the M(1,2,3) sites are almost completely occupied by Mg in richterite. 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 that 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), corresponding to the following local arrangements: MgMgMg–OH–Na–OH:SiSi; MgMgMg–OH–Na–F:SiSi; MgMgMg–OH–Na–F:SiAl; and MgMgMg–OH–□–OH:SiSi in richterite and MgMgMg–OH–Na–OH:SiAl; MgMgMg–OH–Na–F:SiAl; MgMgAl–OH–Na–OH:SiAl; and MgMgAl–OH–Na–F:SiAl in pargasite (□ = vacancy).

You do not currently have access to this article.