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1 2006

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Clinopyroxene from an alkali pyroxenite xenolith, Louená-Oberwiesenthal Volcanic Centre, Bohemian Massif: crystal chemistry and structure

Ulrych D., Niznansky D., Pertlik F., Giester G., Ertl A., Brandstätter F.

Geological Quarterly

2006

Vol. 50, No. 2

257-264

Alkali pyroxenite (and ijolite) xenoliths occur in the Tertiary Loučná-Oberwiesenthal Volcanic Centre associated with the Ohře/Eger Rift. The alkali pyroxenite xenoliths represent fragments of an intracrustal complex with Sr-Nd isotope ratios consistent with mantle sources of HIMU-affinity. The crystal structure of diopside from an alkali pyroxenite xenolith with the formula (...) and the lattice parameters a = 9.773(2), b = 8.886(2), c = 5.308(1) [] and = 105.89(3) [°] was refined to an R-value of 0.025 for 1174 reflections. The mean interatomic distances are: within theMe1-O6 octahedron <2.067> , within theMe2-O8 polyhedron <2.498> . The last value reflects the occupation of this atomic position by significant amounts of Fe2+ and Ti4+. The enlargement determined for the bond length to 1.657 is in accordance with the site population for this position: (Si1.69Al0.31). The molar ratio Fe2+/Fe3+ determined by Mössbauer spectroscopy is equal to 0.786. The AlIV deficiency in T-sites of clinopyroxene of rims is negligible (up to 0.019 a.p.f.u.) restricted to sporadic local electronmicroprobe analyses. The presence of Fe3+ in the T-position of Si- and Al-poor clinopyroxenes was not confirmed by X-ray structural analyses because of its low quantity. Nevertheless, the Mössbauer spectroscopy measurements (isomer shift of 0.36 mm/s) imply that Fe3+ is present only in the Me1-O6 positions.