Abstract

Several diffraction studies on coexisting 1M and 2M1 polytypes have been carried out to date [1, 2, 3, 4, 5, 6] in order to draw informations on their differences by comparing unit layer structure and/or the crystal chemical details of the two polytypic forms. Some of the quoted studies contain implications on polytype formation in micas [1, 2]. In the present work, the crystal chemistry of 1M and coexisting 2M1 micas from Kasenyi (south west Uganda) kamafugite was investigated by Electron Probe Microanalysis, Single C rystal X-ray Diffraction and Mössbauer spectroscopy. The aim of the present study is to compare crystal chemical and unit layer structural features of the coexisting polytypic forms and to make comparisons to literature data. EPMA investigation yielded similar composition for 1M and 2M1 polytypes. They are Ti-rich, F-poor phlogopites with an annitic component, Fetot/(Fetot + Mg), of about 0.14. The room temperature Mössbauer spectrum of the sample yielded three Fe-species: V I Fe2 + = 58(1) % , V I Fe3 + = 23(1) % and I V Fe3 + = 19(1) %. A typical crystal chemical formula is: (K0 .9 6 Na0 .0 4 Ba0 .0 1 )(Mg2 .2 9 Al0 .0 9 Fe2 + 0 .2 1 Fe3 + 0 .0 8 Ti0 .2 8 C r0 .0 4 Ni0 .0 1 )(Si2 .8 7 Al1 .0 6 Fe3 + 0 .0 7 )O1 0 .6 5 F0 .0 5 OH1 .3 0 . Average cell parameters are a = 5.326, b = 9.224, c = 10.231 Å, β = 100.06° for polytype 1M and a = 5.325, b = 9.223, c = 20.206 Å, β = 95.08° for polytype 2M1 . The interatomic distances are similar for the two polytypes and consistent with the relevant site chemistry. The comparison among atomic coordinates of 1M and 2M1 micas from this study and from the literature in the 2M1 setting evidenced a remarkable agreement between all atomic coordinates, with the exception of the y values of the octahedral oxygen atoms. Specifically, the difference between y values was 0.004 in the study samples, 50 times the estimated standard deviations. Similar differences were found for the literature data [1].

Autore Pugliese

• Schingaro Emanuela , Mesto Ernesto

Tutti gli autori

• SCHINGARO E.;MESTO E.;LACALAMITA M.

Titolo volume/Rivista

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Anno di pubblicazione

2011

ISSN

1972-1552

ISBN

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Numero di citazioni Wos

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Settori ERC

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Codici ASJC

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