Short-range order of cations in synthetic amphiboles along the richterite-pargasite join
Ventura, Glancarlo Della; Hawthorne, Frank C.; Robert, Jean-Louis; Delbove, Francois; Welch, Mark F.; Raudsepp, Mati
published: Feb 11, 1999
manuscript accepted: Jul 21, 1998
manuscript received: Jun 15, 1998
ArtNo. ESP147051101012, Price: 29.00 €
Abstract Amphiboles have been synthesized by hydrothermal techniques at 900 °C and 3 kbar at nominal compositions along the join richterite-pargasite [Na(CaNa)Mg5Si8O22(OH)2-NaCa2(Mg4Al)(Si6Al2)O22(OH)2]. Optical microscopy, SEM and powder X-ray diffraction show that in no cases are the run products single phase except for the end-member compositions. HRTEM shows that the amphiboles synthesized are free of CMFs (chain-multiplicity faults). Cell dimensions increase from Pa(0) to Pa(55) and decrease from Pa(80) to Pa(100) except for the ß angle that is linear over the range Pa(0) to Pa(100). The infrared spectrum of richterite in the principal OH-stretching region consists of two bands at 3730 and 3674 cm-1 associated with the local configurations MgMgMg-OH-ANa and MgMgMg-OH-^□, respectively. As the Pa content of the amphibole increases, several new bands appear at different compositions, and the frequencies of these bands are fixed throughout the series. A total of eight bands are used to fit the spectra, although not all bands occur in all spectra; Pa(100) has the most complex spectrum with six bands. Band positions, widths and intensities were derived by least-squares refinement. The number of bands and their relative intensities are related to local configurations in the structure involving Mg and Al at the M(3) site, Si and Al at the T(1) site, and Mg and Al at the M(2) site.