29Si MAS NMR systematics of calcic and sodic-calcic amphiboles

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doi: 10.2138/am-1998-1-209
Authors:Welch, Mark D.; Liu, Shuangxi; Klinowski, Jacek
Author Affiliations:Primary:
Natural History Museum, Department of Mineralogy, London, United Kingdom
Other:
University of Cambridge, United Kingdom
Volume Title:American Mineralogist
Source:American Mineralogist, 83(1-2), p.85-96. Publisher: Mineralogical Society of America, Washington, DC, United States. ISSN: 0003-004X
Publication Date:1998
Note:In English. 35 refs.; 3 tables
Summary:We report the compositional systematics of the 29Si MAS NMR spectra of richterite, [A](Na,K,Rb)1 (Na,Ca,Sr)2Mg5Si8O22(OH,F)2; pargasite, NaCa2Mg4AlSi6Al2O22(OH)2; and fluor-edenite, NaCa2Mg5Si7AlO22F2. [A]Na causes (1) splitting of T1 into two sites, while leaving T2 unsplit, and (2) the Q3 chemical shift to be 2.5 to 3 ppm less negative than Q3 when the A site is empty as in tremolite and magnesiohornblende. The preferential splitting of T1 is explained in terms of ordering of the A cation at Am. The effects of M4 and A-site chemistry upon the richterite spectra aid the assignment of peaks for pargasite and fluor-edenite. The long-range ordering of Al and Si over T1 and T2 sites in fluor-edenite synthesized at 2 kbar, and 1000°C and pargasite synthesized at 1 kbar, and 930°C has been calculated from their 29Si MAS NMR spectra assuming that Al avoidance operates. The extent of long-range order is calculated from the intensities of the Q2(2Al), Q2(1Al), and Q2(0Al) peaks. An equation is derived that allows the extent of long-range Al-Si order to be calculated from 29Si MAS NMR spectra of amphiboles. The spectrum of fluor-edenite is consistent with all [4]Al being at T1 with maximal short-range disorder within the constraints of Al avoidance. The pargasite spectrum is more complex, because there is a probable peak coincidence of Q2(1Al) and Q3(2Al) at -82 ppm that must be considered. The presence of a Q3(0Al) peak in the pargasite spectrum indicates unambiguously that some long-range disorder exists, and this implies that Q3(2Al) groupings also occur. The calculated extent of long-range disorder in pargasite is 55 = 10%. This value is consistent with the single-crystal X-ray data of Oberti et al. (1995a) for natural amphiboles extrapolated to [4]Al = 2 apfu at 900°C. The different long-range Al-Si ordering behavior of flour-edenite and pargasite is explained in terms of the bond-valence requirements of O4. At high temperatures, configurational entropy becomes an important stabilizing factor, and structural distortion around O4 in pargasite and hornblende allows Al into T2, provided that O4 is coupled to Al at an adjacent M2 site, as in pargasite. The results for fluoredenite, which has no [6]Al, show that the low O4 bond-strength sum of the T2AlM2MgM4Ca configuration cannot be accommodated by sufficient structural relaxation, even at 1000°C. Coupling between Al at M2 and T2 is an important control on Al-Si long-range order-disorder.
Subjects:Amphibole group; Chain silicates; Clinoamphibole; Crystal chemistry; Edenite; Electron probe data; Isotopes; NMR spectra; Order-disorder; Pargasite; Richterite; Si-29; Silicates; Silicon; Spectra; Stable isotopes; Synthetic materials
Abstract Numbers:98M/2371
Record ID:1998031930
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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