Infrared and TEM characterization of amphiboles synthesized near the tremolite-pargasite join in the ternary system tremolite-pargasite-cummingtonite

High-resolution transmission-electron microscopy (HRTEM) and infrared spectroscopy (FTIR) analyses have been done on amphiboles near the join tremolite [Ca2Mg5Si8O22(OH)2 = TR]-pargasite [NaCa2Mg4Al3Si6O22(OH)2 = PG] in the ternary system tremolite-pargasite-cummingtonite [Mg7Si8O22(OH)2 = MC] that were synthesized previously by Sharma and Jenkins (1999). Representative samples across the join were examined in detail by HRTEM to document the presence and concentration of chain multiplicity defects (CMFs). There was relatively little change in the defect density with composition, with the tremolitic sample (TREM 23-13) having the highest defect concentration (6%) and the more PG-rich samples having slightly lower CMF concentrations (4–5%). CMFs with multiplicities of 1, 3, 4, 5, and 6 were observed, usually as isolated chains, with the most common being triple-chain slabs. Correction of the bulk composition of the tremolitic amphibole for the presence of these Mg-rich, wide-chain defects reduces the MC content from an apparent value of 8.5 to 4–7.5 mol% MC, depending on which composition is used for the triple-chain defect. The entire amphibole join was examined by FTIR spectroscopy in the OH-stretching region (3000–3800 cm) for the purpose of determining the presence of short-range order. A total of 10 component bands were fitted to the spectra across the join. These bands were assigned to specific cation configurations on the basis of earlier studies of the FTIR spectra of chemically simplified amphibole joins pertinent to this study. The extent of short-range order was qualitatively determined by comparing the observed intensities for groups of related bands, corrected for differences in their molar absorptivities, to their calculated intensities based on random-mixing probabilities. From this exercise, it is observed that the intensities of sodic amphibole configurations are consistently high, tremolite is lowest near the middle of the join, and aluminous amphibole configurations cross over from being higher (at low Al contents) to being lower (at high Al contents) than expected near the middle of the join. These differences between observed and predicted band intensities may reflect the presence of deviations in the thermodynamic activities of amphibole components from those predicted on the basis of random-mixing models. JENKINS ET AL.: IR AND TEM CHARACTERIZATION OF THE TREMOLITE-PARGASITE JOIN 1105 aluminotschermakite join (Hawthorne et al. 2000) to help interpret the spectra in terms of key short-range cation configurations. Another issue that has received considerable attention in recent years is the role that chain-width defects or chain-multiplicity faults (CMFs) play in controlling the chemical composition and bulk thermochemical properties of amphiboles, particularly the presence of either narrow (single-chain) or wider (triple-chain, quadruple-chain, etc.) defects within an otherwise uniformly ordered double-chain silicate (e.g., Ahn et al. 1991; Maresch et al. 1994; Zimmerman et al. 1996). Although there has been some study of the influence of such factors as temperature, fluid pressure, and mineral precursor (i.e., crystalline minerals vs. amorphous oxides vs. gels) in controlling the types or abundances of CMFs within tremolitic amphiboles (Ahn et al. 1991; Maresch et al. 1994; Bozhilov 1997), there is very little information on the control of bulk composition for calcic amphiboles. We report here the results of high-resolution transmission electron microscopy (HRTEM) analysis of some of the same amphibole samples investigated by FTIR spectroscopy. This affords an opportunity to determine (1) how much the chemical composition of these amphiboles is influenced by wide-chain (and presumably Mg-rich) CMFs, and (2) what, if any, dependency the concentration of defects has on bulk composition along this join. MATERIALS AND METHODS