Lattice vibrational modes in synthetic tremolite-Sr-tremolite and tremolite-richterite solid solutions

Abstract

 Powder IR spectra of synthetic richterite-tremolite and Sr-tremolite-tremolite solid solutions were obtained in the spectral range between 1400 and 600 cm⁻¹. Under the consideration of the crystal structure and the Wykoff positions of the atoms in the primitive unit cell, the number, type and symmetry of vibrational modes were deduced. The space group of tremolite C_2h was used as the factor group leading to 16 theoretical stretching vibrations in the IR range caused by the Si₄O₁₁ ^∞-ribbon. The energy of the internal vibrations of the Si₄O₁₁ ^∞-ribbon is a function of the relative bond strengths and masses of nearby ions. For the amphiboles a one-mode behavior was observed for all the Si-O, Si-O-Si and O-Si-O stretching vibrations, indicating no clustering in the two solid solution series. In both solid solution series the vibrational energy of the stretching vibrations is a linear function of composition. In the system richterite-tremolite a shift of the stretching frequencies of the Si₄O₁₁ ^∞-ribbon over the whole compositional range of up to 30 cm⁻¹ was observed. In contrast, for Sr-tremolite-tremolite the maximum shift was only 5 cm⁻¹. These quite small band shifts allow the (Si₄O₁₁)^∞-ribbon to be treated as an isolated entity for factor group analysis. Nevertheless, by the two exchange mechanisms, Ca(M4) ? Sr(M4) and □(A) Ca(M4) ? Na(A)Na(M4), the FWHHs increased and the amplitudes decreased, indicating a slight distortion of the ribbon. For Sr-tremolite-tremolite only a linear expansion of the lattice was observed. In the series richterite-tremolite individual bond angles of the SiO₄ tetrahedra are additionally changed, causing the higher energy shift of the bands. The strongest and sharpest bands were observed for the end member tremolite. The one-mode behavior of the Si₄O₁₁-double chain indicates that there is no short-range order of Na/Ca and Ca/Sr at the M4 sites of these amphiboles.