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

Abstract

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 (Ca0.95Na0.04) (Mg0.65 Fe Fe Ti0.10 4+ 0 13 3 0 10 2 . . + + Al0.01) (Si1.69Al0.31) O6 and the lattice parameters a = 9.773(2), b = 8.886(2), c = 5.308(1) [] and b = 105.89(3) [°] was refined to an R-value of 0.025 for 1174 reflections. The mean interatomic distances are: within the Me1-O6 octahedron< 2.067>>gt; , within the Me2-O8 polyhedron< 2.498>>gt; . The last value reflects the occupation of this atomic position by significant amounts of Fe2+ and Ti4+ . The enlargement determined for the< T-O>>gt; 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 electron microprobe 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.