The crystal structure of nonmetamict Nb-rich zirconolite-3<i>T</i> from the Eifel paleovolcanic region, Germany

Abstract

Abstract The crystal structure of a Nb-rich zirconolite-3 T from the Laach Lake volcano, Eifel, Germany, was studied by single-crystal XRD ( R =0.0295). The mineral is trigonal, P 3 1 21; unit-cell dimensions are: a= 7.3095(2), c =16.9604(5) Å, V =784.78(4) Å 3 . The empirical formula based on 14 O atoms ( Z =3) is Ca 1.28 Ce 0.31 La 0.14 Nd 0.12 Pr 0.06 Th 0.16 Zr 1.94 Hf 0.04 Ti 1.74 Nb 1.22 Fe 0.72 Mn 0.28 O 14 . The crystal-chemical formula based upon the structural model is: VIII (Ca 0.70 Ce 0.30 ) VIII (Ca 0.58 Ce 0.32 Th 0.10 ) VII (Zr 0.88 Th 0.04 Hf 0.02 □ 0.06 ) 2 VII (□ 0.94 Zr 0.06 ) 2 VI (Ti 0.72 Nb 0.26 Zr 0.02 ) VI (Ti 0.50 Nb 0.47 Zr 0.03 ) 2 IV (Fe 0.355 Mn 0.145 ) 2 O 14 ( Z =3).