STRUCTURAL AND CHEMICAL INVESTIGATION OF PERISTERITES BY TRANSMISSION ELECTRON-MICROSCOPY (TEM) AND X-RAY-DIFFRACTION

Peristerites from Steiermark (Austria) and from Burma have been studied in detail by transmission electron microscopy (TEM) and X-ray diffraction to obtain the compositions of their coexisting lamellae. A strong influence of lamellar periodicity on the measured lattice parameters was found in peristerites with different lamellar spacings. Precise determination of the compositions of the lamellae was only possible for the Burma peristerite, which has a large periodicity of 170 +/- 30 nm. Qualitative energy dispersive X-ray analyses of the lamellae in this sample, carried out with an analytical TEM, demonstrate a significant enrichment of the Or-content in the smaller An-rich lamellae. In the light of this result, an unambiguous determination of the lamellar compositions is obtained from the lattice parameters of the two phases. Powder diffraction patterns and single-crystal diffraction patterns of this sample show similar results. The compositions of the lamellae of the Burma peristerite, which has a bulk composition of Ab(91.4)An(6.8)Or(1.8), were determined as Ab(98)An(2)Or(0) and Ab(74)An(18)Or(8) With an uncertainty of 2 mol%. In contrast with the Burma peristerite, lattice parameters of the Steiermark peristerite are strongly influenced by lattice distortion resulting from the small lamellar periodicity of 50 +/- 10 nm. X-ray data of the exsolved phases include strains arising from the adaptation of their lattice parameters to each other and are not suitable for estimating the compositions of the lamellae. Controversy arising from determination of the compositions of peristerite lamellae from lattice parameters may be due to an enrichment of the Or-content in the An-rich phase, which has not been taken into account in previous studies.