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Crystal chemistry of three tourmalines by SREF, EMPA, and SIMS (Articolo in rivista)

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Crystal chemistry of three tourmalines by SREF, EMPA, and SIMS (Articolo in rivista) (literal)

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Camara F.; Ottolini L.; Hawthorne F.C. (2002)
Crystal chemistry of three tourmalines by SREF, EMPA, and SIMS
in The American mineralogist; Mineralogical Society of America, Washington (Italia)
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This paper presents complete analytical data on three specimens of tourmaline, a complex silicate mineral that shows various chemical compositions and is present in a wide range of geological environments. The specimens come from the Harvard Museum and are part of a selection of mineral samples chosen with the aim of produce high-quality standards. Despite its high frequency in Nature and due to the presence of light elements (i.e. H, Li, B, F), complete chemical characterisation is not usually achieved. Excellent agreement between different techniques (SREF, EMPA and SIMS) has been found. Comparison with previous data on the same material, has revealed the presence of analytical problems that should be taken into account in particular when dealing with the analysis of light elements. Further, high resolution X-ray diffraction data has revealed fine features in the electron density related to local cation ordering described for the first time in literature. (literal)

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The crystal structures of three tourmaline have been refined to R-indices of 1.32.2% using X-ray intensity data. The crystals were analyzed by electron- and ion-microprobe techniques for all major and minor elements in the crystals. Unit formulae were calculated on the basis of 31 anions (O, OH, F) and the Fe3+/(Fe2+ + Fe3+) ratio was calculated for electroneutrality. The refined site-scattering values and the observed and distances were used to assign site populations that are in accord with the unit formulae calculated from the electron- and ion-microprobe compositions. The B contents are equal to 3.0 apfu (atoms per formula unit) within experimental error. In two of the crystals, (OH + F) = 4.0 apfu. However, the third crystal has (OH + F) = 3.36 apfu and O2 is dominant at the W(O1) site. It is an oxy tourmaline. Non-spherical electron-density was observed at the X site, suggesting that there is some positional disorder at this site associated with occupancy of X by Ca and Na, possibly coupled with variable anion occupancy of the O1 site. (literal)

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F. Cámara a), L.Ottolini a), F.C. Hawthorne b). a) CNR-IGG Sez. Pavia, via Ferrata 1, 27100 Pavia, Italy; b) Department of Geological Sciences, University of Manitoba, Winnipeg, MB R3T 2N2, Canada (literal)

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The crystal structures of three tourmaline crystals: (Na0.49 K0.01 Ca0.48) (Mg1.35 Fe2+0.94 Fe3+0.49 Ti0.20)(Al4.58 Fe3+0.62 Mg0.80) (Si5.99 Al0.01) O18 (BO3)3.03 (OH)3.18 F0.18 O0.64; (Na0.64 K0.01 Ca0.03) (Mn0.18 Fe2+1.71 Al0.88 Li0.11 Zn0.03 Ti0.07) (Al5.67 Fe3+0.28 Mg0.05) (Si5.76 Al0.24) O18 (BO3)2.99 (OH)3.96 F0.17; (Na0.81 K0.01 Ca0.01) (Mn0.02 Mg0.61 Fe2+0.90 Al0.80 Li0.70 Zn0.01 Ti0.06) Al6.00 (Si5.97 Al0.03) O18 (BO3)2.93 (OH)3.42 F0.55 O0.03, have been refined to R-indices of 1.32.2% using X-ray intensity data. The crystals were analyzed by electron- and ion-microprobe techniques for all major and minor elements in the crystals. Unit formulae were calculated on the basis of 31 anions (O, OH, F) and the Fe3+/(Fe2+ + Fe3+) ratio was calculated for electroneutrality. The refined site-scattering values and the observed and distances were used to assign site populations that are in accord with the unit formulae calculated from the electron- and ion-microprobe compositions. The B contents are equal to 3.0 apfu (atoms per formula unit) within experimental error. In two of the crystals, (OH + F) = 4.0 apfu. However, the third crystal has (OH + F) = 3.36 apfu and O2 is dominant at the W(O1) site. It is an oxy tourmaline. Non-spherical electron-density was observed at the X site, suggesting that there is some positional disorder at this site associated with occupancy of X by Ca and Na, possibly coupled with variable anion occupancy of the O1 site. (literal)

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