Crystallographic study of the structural dissymmetrization of the garnets of grossullar-andradite isomorphous series.

[P. Tančić]

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Geonauke - Kristalografija / Geosciences - Crystallography

U ovoj Doktorskoj disertaciji proučavano je pet različitih zona makroskopski zonarnog grandita sa lokaliteta Meka Presedla (Kopaonik, Srbija). Pomoću hemijske i elektronske analize mikroprobe preračunati su relativno visoko homogeni sastavi zona, t.j.: Grs64±1Adr36±1Sps2 (A), Grs62±1Adr38±1Sps2 (B), Grs59±2Adr40±2Sps2 (C), Grs58±2Adr41±2Sps2 (D) i Grs58±1Adr41±1Sps2 (E). Glavne promene u sastavima izražene formulom: Ca2,93-2,97Mn0,05-0,06Mg0,00-0,01Al1,14-1,26Fe3+0,72-0,83Ti0,00-0,02Si2,97-3,02O12 ukazuju da je jedina značajnija varijacija ustanovljena samo kod Al-Fe3+ raspodela u oktaedarskom položaju. Ovakav hemijski sastav je potvrđen i sa matematičkim proračunima iz IC i Raman analiza. Urađena je takođe i karakterizacija zona Raman spektroskopskom metodom, ukazujući na dva tipa promena. Brojne strukture su određene Rietveld metodom. Ustanovljena je desimetrizacija u Fddd ili R‾3c prostornim grupama, pri čemu je prva prostorna grupa znatno verovatnija. Na taj način, ovi rezultati se mogu svrstati u samo nekoliko publikovanih radova u svetu u kojima je desimetrizacija grandita nesumnjivo dokazana, a prvi rad takve vrste sa podacima koji su dobijeni rendgenskom difrakcionom analizom praha. Pojedina značajna odstupanja u okviru među-jonskih rastojanja ukazuju da se rotacija tetraedra, poliedarske distorzije i najverovatnije valence poliedara razlikuju između zona. Uslovi postanka su procenjeni na: t=~650-690 (715) oC, p=2-3 kbara i XCO2 ≥ 0,6. Dobijeni rezultati argumentovano ukazuju da je desimetrizacija prouzrokovana pre svega termodinamikom i kinetikom postanka grandita, a ne njegovim sastavom.

In this Doctoral Dissertation it was studied five different zones of the macroscopically zoned grandite from the Meka Presedla locality (Kopaonik Mt., Serbia). With the chemical and electron microprobe analyses it was recalculated relatively high homogeneity composition levels of the zones, i.e.: Grs64±1Adr36±1Sps2 (A), Grs62±1Adr38±1Sps2 (B), Grs59±2Adr40±2Sps2 (C), Grs58±2Adr41±2Sps2 (D) and Grs58±1Adr41±1Sps2 (E). Main compositional variations represented by formulae: Ca2,93-2,97Mn0,05-0,06Mg0,00-0,01Al1,14-1,26Fe3+0,72-0,83Ti0,00-0,02Si2,97-3,02O12 indicate that the only significant variation was established for Al-Fe3+ distribution within the octahedral position. Such chemical composition was also confirmed with mathematical calculations from IR and Raman analysis. Also, zones were characterized with Raman spectroscopical method, indicating two types of variation. Numerous structures were refined with Rietveld method. It was established dissymmetrization into Fddd or R‾3c space groups, whereby is the first one space group with quite more probability. In this way, these results may be seen in only a few published papers in the World in which the grandite dissymmetrization was undoubtedly proven, and the first work of its kind with the data obtained by X-ray powder diffraction analysis. Some major differences within the inter-ionic distances indicate that the tetrahedron rotations, polyhedral distortions and most probably valences of the polyhedrons differs accross the zones. Formation conditions were estimated to: t=~650-690 (715) oC, p=2-3 kbars and XCO2 ≥ 0.6. The obtained results reasonably indicate that dissymmetrization was caused primarily by thermodynamic and kinetic of the grandite formation, and not by composition.

Macroscopically zoned grandite, chemical analysis, electron microprobe analysis, Raman, Rietveld, structure, dissymmetrization, space group

Makroskopski zonarni grandit, hemijska analiza, elektronska analiza mikroprobe, Raman, Rietveld, struktura, desimetrizacija, prostorna grupa

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