Structural and chemical characterization of synthetic (Na,K)-richterite solid solutions by EMP, HRTEM, XRD and OH-valence vibrational spectroscopy

Abstract

The composition and structure of synthetic (Na,K)-richterites have been 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 endmembers richterite and K-richterite alone, but tremolite and Mg-cummingtonite components are also present in considerable amounts. HRTEM observations show that the amphiboles are structurally well ordered. Only a very few chain multiplicity faults are present. XRD examination reveals lattice parameters of 9.9055 A, 17.9844 A, 5.2689 A and 104.212° for richterite and 10.0787 A, 17.9877 A, 5.2715 A and 104.878° for K-richterite endmembers. The unit cell volumes are 909.90 A3 and 923.61 A3 for richterite and K-richterite, respectively. The lattice parameters a and β for K-richterite are considerably larger than those published previously implying that those were not determined for pure K-richterite. 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 determinations. Using synthetic tremolite as a standard the vacancy concentration on the A-site of the synthetic (Na,K)-richterites was determined quantitatively by FTIR-spectroscopy. The OH-stretching vibration of this synthetic tremolite is at 3674.5 cm−1. It is assigned to a local coordination with 3 Mg (2 M1+M3) as nearest neighbors and with 2 Ca (M4) as next nearest neighbors. A well resolved band with a smaller intensity is located at 3669.2 cm−1, which is attributed to a configuration including Ca+Mg on M4 instead of only Ca.