The determination of the cummingtonite content of synthetic tremolite
Gottschalk, Matthias; Andrut, Michael; Melzer, Stefan
European Journal of Mineralogy Volume 11 Number 6 (1999), p. 967 - 982
published: Nov 29, 1999
manuscript accepted: May 10, 1999
manuscript received: Nov 30, 1998
ArtNo. ESP147051106006, Price: 29.00 €
Abstract Amphiboles of predominantly tremolite composition have been synthesized at 670-800 °C and 200-700 MPa in the system CaO-MgO-SiO2-H2O. The tremolite crystals were large enough to be analyzed by electron microprobe. The run products were also studied by scanning electron microscopy, high resolution electron microscopy, X-ray diffraction (Rietveld analysis) and IR spectroscopy. The electron microprobe analyses resulted in average compositions between tr87cum13 and tr99cum1. The cummingtonite contents determined by electron microprobe varied by up to ±7 mol% within each run. Investigations using high resolution electron microscopy revealed that the tremolite-cummingtonite solid solutions were highly ordered and chain multiplicity faults were rare. Although the lattice parameters α, b and β are expected to be a function of the cummingtonite content, no correlations were observed between the compositions derived by electron microprobe and the lattice parameters. The energy and absorbance of the OH stretching vibration is a function of M4 site occupancy. Therefore, the cummingtonite content was determined precisely by IR spectroscopy. At least 3 different bands at distinct energies were identified: 3674.6 cm-1 for the CaCa-CaCa and 3669.3 and 3672.2 cm-1 for the MgCa-CaCa and CaCa-MgCa configurations, respectively. The relative integral absorbances showed that all synthesized amphiboles had cummingtonite contents lower than 6 mol%. In some cases nearly pure tremolite was synthesized (tr99cum1). This amount is considerably less than the 10 mol% cummingtonite content (tr90cum10) often claimed for synthetic tremolite. The compositions derived by IR spectroscopy correlate with the lattice parameters. Using these correlations the lattice parameters of pure tremolite were extrapolated to be a = 9.8354(18) Å, β = 18.0562(14) Å, c = 5.2768(6) Å, β = 104.74(2)°, V= 906.3(2) Å3.