American Mineralogist; July 2000; v. 85; no. 7-8;
p. 993-1000
© 2000 Mineralogical Society of America
Structure and twinning of tetragonal Ca3Mn2Ge3O12 garnet
Stefan Heinemann1,2,* and
Ronald Miletich1,
1 Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany
2 Laboratoire de Structure et Propriétés de l’Etat Solide, Université des Sciences et Technologies de Lille, Cité Scientifique, 59655 Villeneuve d’Ascq Cedex, France
Single crystals of tetragonal Ca3Mn2Ge3O12 garnet (space group symmetry I41/a, a = 12.3098(7) Å, c = 12.3277(9) Å) were characterized by X-ray diffraction and transmission electron microscopy. Their structure is topologically isosymmetric to tetragonal high-pressure garnets such as majorite, displaying the same two distinct types of macroscopic twin mechanisms. Twinning occurs as pseudo-merohedral ferroelastic twin lamellae with preferred orientation of the twin-domain boundaries parallel to {101}tet, whereas merohedral ferrobielastic twin domains occur without any orientational preference. The crystal structure was determined from single-crystal X-ray diffraction data of a crystal fragment completely free of pseudo-merohedral twin domains. It shows two different Jahn-Teller distorted MnO6 octahedra, with a different orientation of the axis of polyhedral elongation. The ordering scheme of these Jahn-Teller distorted octahedra follows in an alternating pattern the densest rod packing, and the cooperative effect of the electronically induced octahedral distortion was found to be responsible for the cubic-to-tetragonal symmetry breaking in Ca3Mn2Ge3O12 garnet. The extent of polyhedral distortion indicates a partially dynamic character of the Jahn-Teller effect. The distortion of the tetrahedral T2 sites controls the cooperative effect of the lattice strain induced by the Jahn-Teller distortion of the Mn3+ octahedra and thus is responsible for the overall lower symmetry.
Copyright © 2009 by Mineralogical Society of America
