Some Crystallo-chemical Relations of Nephelines and Feldspars on Stjerny, North Norway

Chemical, X-ray, and optical data are given for coexisting nephelinesand potassium feldspars separated from a nepheline syenite,and of nepheline and albite from pegmatites. Li, Rb, Cs (detectedonly in one nepheline), Ba, and Sr were determined, in additionto the major elements. (Pb and Tl were not detected by the spectrographicmethod used.) Some peculiarities of the mineral structures which are consideredto be important for the partition of elements between the coexistingphases are discussed. The importance of vacancies in nephelines,and their probable restriction to the large cavities in thestructure, is emphasized. It is pointed out that since low concentrations of Rb, Cs, Pb,and Tl are found in both nepheline and feldspar, they cannotbe caused by an unfavourable nepheline structure. They are eithera property of the parent magma or are the result of the physicalconditions of crystallization (i.e. escaping gas phase). Thelow concentrations of Sr, and especially of Ba, in nephelinesare in contrast to the high contents of these elements in thecoexisting feldspars. This is explained by considering the nephelinestructure. The unit cell contains eight structural sites forthe large cations (Na, K, Ca), and there are eight valenciesto satisfy. Six of the sites are smaller than the other two,and preferentially incorporate Na (and Ca) ions. The two largersites will tend to remain as vacant sites. Those which haveto be filled in order to satisfy the charge balance, will preferentiallyincorporate the large univalent ions (K and H₃O). The higher-chargedions (Ba² and Sr²) will pull the anions closer together andreduce the size of the site. They are, therefore, rejected fromthese positions. When nephelines coexist with feldspar, theywill incorporate the maximum amount of Si possible at the temperatureof formation. The substitution of Al for Si simultaneously withthe entry of a divalent cation (as in feldpspar) is, therefore,not possible except in non-feldspathic, extremely desilicifiedrocks. Thus, the entry of a divalent cation will not effectivelyclose the vacant sites. The somewhat higher contents of Sr relativeto Ba in the nephelines is because Sr is more easily incorporatedin the small sites (with Na and Ca).     

PULSE DISTORTION IN CAUSTIC REGIONS1

It is well known that there is a phase shift of π associated with the propagation of a seismic pulse through a focus. But this simple behaviour is true only if the two points at which the pulse is observed lie sufficiently far from the focus, one on either side. Near the focus matters are more complicated and the shape of the pulse may be severely distorted because of diffraction. To examine this problem we consider a pulse generated by a point source and focused by a spherical reflector. Using a general combined method of ray tracing and diffraction, we compute seismograms for receivers lying both inside and outside the focal region, and determine how the shape of the pulse changes on propagation through the focus. To assist the physical interpretation of the numerical results, we also present relevant exact and asymptotic results for comparison.