Gem amphiboles from Mogok, Myanmar: crystal-structure refinement, infrared spectroscopy and short-range order–disorder in gem pargasite and fluoro-pargasite. (14th September 2018)

• Record Type: Journal Article
• Authors: Day, Maxwell C.; Hawthorne, Frank C.; Susta, Umberto; Della Ventura, Giancarlo; Harlow, George E.
• Is Part Of: Mineralogical magazine. Volume 83:Number 3(2019)
• Page Start: 361
• Page End: 371
• Publication Date: 2018-09-14
• Subjects: pargasite, -- amphibole, -- gem, -- crystal structure, -- infrared spectroscopy, -- Myanmar, -- short-range order; Mineralogy -- Periodicals; Mineralogy -- Great Britain -- Periodicals; 549.05
• DOI: 10.1180/mgm.2018.149
• Languages: English
• ISSNs: 0026-461X

Abstract

Abstract: The crystal structures of six gem-quality pargasites and fluoro-pargasites from Mogok, Myanmar, space group C 2/ m, Z = 2, have been refined to R 1 indices of 2.20–2.90% using Mo K α 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. T Al occurs at both the T (1) and T (2) sites but is strongly ordered at T (1). [6] Al is partly disordered over the M (2) and M (3) sites but does not occur at the M (1) site. A Na 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.