JARLITE: ORDER-DISORDER STRUCTURE SUGGESTED BY OPTICS, RESULTS OF NEW ANALYSES AND EMPIRICAL FORMULAE
ABSTRACT Results of electron-microprobe analyses, X-ray data and density determinations ofjarlite and "meta-jarlite" from Ivigtut, Greenland, are presented. Common jarlite, in crystals and grains, is composite; it has a core with straight boundaries and a mantle. The core shows a 2Voof <...
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| Format: | Text |
| Language: | English |
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1992
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1037.2471 http://rruff.info/uploads/CM30_449.pdf |
| Summary: | ABSTRACT Results of electron-microprobe analyses, X-ray data and density determinations ofjarlite and "meta-jarlite" from Ivigtut, Greenland, are presented. Common jarlite, in crystals and grains, is composite; it has a core with straight boundaries and a mantle. The core shows a 2Voof <40"; the mantle, in optical continuity with the core, shows a 2Zo of approximately 80o. The core is rich in Sr and Ba, but nearly devoid of Mg and K. In the mantle, Mg and K are present in significant amounts. "Meta-jar1ite", always developed as dendrites, is chemically similar to the mantle jarlite. Jarlite that is not composite, from the stenonite paragenesis, with2V, between 5 and l0o, has a composition similar to that of jarlite in the core. Empirical formulae calculated from the new analytical data on the basis of 68 anions give 30 cations, in agreement with the nominal formula: Na2(Sr12Na)Mg2Al12F6a(OH)a. The X-ray data, density determinations and calculated molecular weights support a formula based on 68 anions, but the calculated formula content cannot be accommodated in the structure in accordance with the model proposed. The empirical formula for jarlite in the core may be written Na2(Sr,Ba,Na)16A112F6, assuming disorder between Mg and Sr sites. The empirical formulae for jarlite mantle and "meta-jarlite" show 1.5 to 2 atoms of Mg, and 14.5 to 15 atoms for the Sr sites. The core-mantle arrangement of jarlite and the different compositions suggest a shift during crystallization, from a disordered toward an ordered structure. The jarlite from the stenonite paragenesis seems to represent the disordered structure only. Keywords: jarlite, electron-microprobe analyses, X-ray data, density, Sr aluminofluoride, empirical formulae, order-disorder structure, Ivigtut, Greenland. Sovtrlernr Nous pr6sentons les resultats d'analyses d la microsonde 6lectronique, des donndes de diffraction rayons X et des d6terminations de densit6 de la jarlite et de la "m€ta-jarlite" de lvigtut, au Grodnland. La jarlite commune, en cristaux et en ... |
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