A 94-layer long-period mica polytype was studied by transmission electron microscopy (TEM). This is the longest periodicity found up to now in micas (c ≈ 95.9 nm). It was observed in a fragment of a Mg-rich annite (biotite) crystal from dacite rocks of Džep, Serbia. The crystal region containing it extends about 800 nm along 1/c*. One-dimensional lattice fringe images obtained by bright-field (BF) illumination allowed identification of the very long-period polytype. The latter was characterized by selected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). It is an inhomogeneous polytype belonging to the subfamily-A, based on the 2M1 structural series. Its 94-layer stacking sequence has been determined. The fringe contrasts of the BF images were correlated to the stacking sequence determined from HR images. The correlation verified that the same sequence occurred eight consecutive times. Analytical electron microscopy (AEM) revealed that the chemical composition of the 94-layer mica polytype is similar to that previously observed in randomly stacked and faulted areas of the same crystal. No remarkable chemical variation occurs between the 94-layer polytype and its adjacent crystal regions, the latter containing non-periodic stacking faults.

The chemical and structural features of a natural spinel sensu stricto (s.s.) sample were studied by a multi-analytical approach, including electron microprobe analysis (EMP), Fourier transform infrared spectroscopy (FTIR), and single crystal X-ray diffraction structural refinement (SREF). The sample, coming from impure dolomitic marbles of Pegu (Myanmar), has an anomalous chemistry with an Mg-content exceeding that of the ideal formula. In addition, a chemical zoning along a line-scan of EMP analyses was observed, with Mg and Al amounts showing opposite trends. The comparatively high and low concentrations, respectively, of divalent and trivalent cations lead to a deficit of positive charges. Thus, the requirement of neutrality of global charges for crystal structures appears to be violated, in this case. The possible reasons accounting for the anomalous chemistry are discussed. Based on the combined EMP, FTIR and SREF results, it is concluded that anion vacancies can adequately compensate for the observed deficit of positive charges. Thus, the analysed sample is a defect spinel. This is the first report of anion vacancies in a natural spinel s.s. With reference to the ideal formula MgAl2O4, the formation of anion vacancies, coupled to an excess of Mg and a deficiency of Al, may be described by the substitution mechanism 2Mg2++V□→2Al3++O2–, where V□ represents an oxygen vacancy.

A multi-analytical approach using electron microprobe analysis, X‑ray structural refinement, and optical absorption spectroscopy was applied to characterize short-range and long-range structures of synthetic spinel single crystals along the MgAl2O4-CuAl2O4 solid-solution series. Site populations, derived from the results of site-scattering refinement and stereochemical analysis, show that the tetrahedrally coordinated site (T) is mainly populated by Mg and Cu2+, while the octahedrally coordinated site (M) is dominated by Al. Crystals also show a significant degree of inversion, i.e., occurrence of Al at T counterbalanced by occurrence of divalent cations at M, which increases slightly from 0.24 to 0.29 for the highest Cu2+ contents. Short-range information derived from optical spectra suggests that the local TCu2+-O distances remain constant at increasing Cu2+ content, whereas local MCu2+-O distances are ca. 0.02 Å shorter in Cu-poor MgAl2O4 spinels as compared to MCu2+-O distances in end-member CuAl2O4. The observed splitting of an absorption band, at ca. 7000 cm–1, caused by electron transitions in TCu2+ as well as the anomalous broadness of an absorption band, at ca. 13 500 cm–1, caused by electron transitions in MCu2+ indicates the occurrence of local Jahn-Teller distortions at T and M. Long-range information, however, shows no violation of Fd3m symmetry. From refinements of our single-crystal XRD data we could for the first time derive for a cubic spinel phase a MCu2+-O distance of 2.080 Å and a TCu2+-O of 1.960 Å. The very limited variations in the unit-cell parameter a from 8.079 to 8.087 Å are mainly related to the disordering of Al. Because of the very similar size of Cu2+ and Mg at the T and M sites, the spinel structure responds to the Cu2+ → Mg substitution by increasing cation disordering in such a manner that mean M-O distances remain constant and the mean T-O distances decrease slightly. This results in increasing length of shared octahedral edges and thereby increase of the octahedral cation-cation repulsion. In line with other studies, the importance of steric factors for controlling the cation distributions in the spinel structure is demonstrated to be valid also in the MgAl2O4-CuAl2O4 solid-solution series.