Scapolite : alkali atom configurations , antiphase domains , and compositional Yariations
Electrostatic energy calculations on an intermediate scapolite indicate that Na+ should prefer to be adjacent to Clanions and Ca2* to be adjacent to CO3radicals. The calculations also indicate that short-range ordering of NarCl and CaaCO3 clusters is energetically favored and might give rise to antiphase domains. Electrical neutrality considerations on the anion/radical site in scapolite suggest that the unusual compositional variations observed in the marialite-meionite series are the result of crystallographic constraints on the two possible independent exchange reactions in scapolite: NaCICaCO-3 and NaSie6-Ai. The compositions of naturally occurring scapolites may be explained in terms of the scapolite composition that represents local charge balance between Ca2*, Na* cations and Cl-, COIanions.
High Resolution Electron Microscopy of Silicates
Imaging of crystal structures of minerals clearly shows details within ideally periodic regions, even at the unit cell scale. A variety of crystal imperfections can be observed at point-to-point resolutions down to 3A-intermediate between bond lengths and unit cell dimensions. Fluctuations in both structure and composition can be observed in volumes up to 14 orders of magnitude smaller than is possible by standard single crystal X-ray techniques. We have obtained crystal structure images of the following silicate structure types: "ring" structures (beryl and cordierite), isolated and paired tetrahedra (idocrase), single and double chains (orthoenstatite and hornblende), framework (scapolite), and sheet (muscovite). Regular and perturbed regions containing defects are clearly differentiable. Antiphase domains (hornblende) and stacking faults (enstatite) as well as the effects of radiation damage produce recognizable structural effects, only some of which are reflected in the electron diffraction patterns. An example of the usefulness of the technique is the resolution of small regions (several unit cells wide parallel to [100D of clinoenstatite intergrown within orthoenstatite. The technique of high resolution electron microscopy has great potential for mineralogy and geology. Solid solution series may well be resolvable into stoichiometric phases differing only slightly in both composition and/or structure. Homologous phases, potentially resolvable by electron microscopy, have the possibility of providing far more sensitive mineralogical indicators to past history and environments than currently exist.
The Russian language maintenance and language contacts
In our introductory article, we outline the main sociolinguistic features of Russian as a heritage language of post-Soviet immigrants in European settings and beyond. We offer a general overview of the evolution of Russian as a global language, with a particular focus on its geodemographics and economic and social value as a lingua franca . Based on this, we analyse the main principles defining the maintenance of Russian as a language of migration and as a heritage language in different countries, and emphasise the most important questions that still need to be addressed in this field of research. The main objective of this special issue is to combine the most recent research on the vitality of different languages of post-Soviet republics in new political milieu, with a particular focus on European and Asian countries, but there are other objectives as well. We propose to explore the factors that have either favoured or hindered the maintenance and transmission of languages of post-Soviet immigrants and repatriates, and how these sociolinguistic processes become evident in language vitality on both private and public levels. Our special issue primarily addresses the questions of family language policy, new language contacts and their management, and linguistic landscape in heritage speakers, diasporas and their new settings in Europe, Asia and the US.