STRUCTURAL AND CHEMICAL CHARACTERIZATION OF SYNTHETIC (NA,K)-RICHTERITE SOLID-SOLUTIONS BY EMP, HRTEM, XRD AND OH-VALENCE VIBRATIONAL SPECTROSCOPY

The composition and structure of synthetic (Na,K)-richterites have bee n characterized by EMP, HRTEM, XRD and FTIR methods. Despite the fact that the syntheses were done on bulk compositions along the richterite -K-richterite binary, EMP analyses and FTIR spectra indicate that the amphiboles are not simple solid solutions of the two richterite endmem bers richterite and K-richterite alone, but tremolite and Mg-cummingto nite components are also present in considerable amounts. HRTEM observ ations show that the amphiboles are structurally well ordered. Only a very few chain multiplicity faults are present. XRD examination reveal s lattice parameters of 9.9055 Angstrom, 17.9844 Angstrom, 5.2689 Angs trom and 104.212 degrees for richterite and 10.0787 Angstrom, 17.9877 Angstrom, 5.2715 Angstrom and 104.878 degrees for K-richterite endmemb ers. The unit cell volumes are 909.90 Angstrom(3) and 923.61 Angstrom( 3) for richterite and K-richterite, respectively. The lattice paramete rs a and beta for K-richterite are considerably larger than those publ ished previously implying that those were not determined for pure K-ri chterite. The positions of the characteristic OH-stretching vibrations in the IR for sodium potassium (3729.8-3734.8 cm(-1)) and vacancies ( 3671.1-3675.4 cm(-1)) on the A-site are in agreement with earlier dete rminations. Using synthetic tremolite as a standard the vacancy concen tration on the A-site of the synthetic (Na,K)-richterites was determin ed quantitatively by FTIR-spectroscopy. The OH-stretching vibration of this synthetic tremolite is at 3674.5 cm(-1). It is assigned to a loc al coordination with 3 Mg (2 M-1 + M-3) as nearest neighbors and with 2 Ca (M-4) as next nearest neighbors. A well resolved band with a smal ler intensity is located at 3669.2 cm(-1), which is attributed to a co nfiguration including Ca+Mg on M-4 instead of only Ca.