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.
Heritage Languages: In the 'Wild' and in the Classroom
Heritage speakers are people raised in a home where one language is spoken who subsequently switch to another dominant language. The version of the home language that they have not completely acquired – heritage language – has only recently been given the attention it deserves from linguists and language instructors. Despite the appearance of great variation among heritage speakers, they fall along a continuum based upon the speakers’ distance from the baseline language. Such a continuum-based model enables researchers and instructors to classify heritage speakers more accurately and readily. This article discusses the results of research on lower-proficiency speakers, identifying recurrent features of heritage languages in phonology, morphology, and syntax. Preliminary results indicate that different heritage languages share a number of structural similarities; this finding is important for the understanding of general processes involved in language acquisition. The article also presents implications of the main findings for language education and identifies areas needing further study.