Short-range order of cations in synthetic amphiboles along the richterite-pargasite join

Amphiboles have been synthesized by hydrothermal techniques at 900 degrees C and 3 kbar at nominal compositions along the join richterite-pargasite [N a(CaNa)Mg5Si8O22(OH)(2)-NaCa2(Mg4Al)(Si6Al2)O-22(OH)(2)]. Optical microscop y, SEM and powder X-ray diffraction show that in no cases are the run produ cts single phase except for the end-member compositions. HRTEM shows that T he amphiboles synthesized are free of CMFs (chain-multiplicity faults). Cel l dimensions increase from Pa(0) to Pa(55) and decrease from Pa(80) to Pa(1 00) except for the beta angle that is linear over the range Pa(0) to Pa(100 ). The infrared spectrum of richterite in the principal OH-stretching regio n consists of two bands at 3730 and 3674 cm(-1) associated with the local c onfigurations MgMegMg-OH-Na-A and MgMgMg-OH-(A)square, respectively. As the Pa content of the amphibole increases, several new bands appear at differe nt compositions, and the frequencies of these bands are fixed throughout th e 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-squ ares refinement. The number of bands and their relative intensities are rel ated 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.