ORIGINAL PAPER Experimental multipole-refined and theoretical charge density study of LiGaSi 2 O 6 clinopyroxene at ambient conditions R. Bianchi Æ A. Forni Æ F. Ca ´mara Æ R. Oberti Æ H. Ohashi Received: 27 February 2007 / Accepted: 1 May 2007 / Published online: 1 June 2007 Ó Springer-Verlag 2007 Abstract The synthetic LiGaSi 2 O 6 clinopyroxene is monoclinic C2/c at room-T. Its experimental electron density, q(r), has been derived starting from accurate room-T single-crystal diffraction data. Topological analysis confirms an intermediate ionic-covalent character for Si–O bonding, as found by previous electron-density studies on other silicates such as diopside, coesite and stishovite. The non-bridging Si–O bonds have more covalent character than the bridging ones. The Ga–O bonds have different bonding characters, the Ga–O2 bond being more covalent than the two Ga–O1 bonds. Li–O bonds are classified as pure closed-shell ionic interactions. Similar to spodumene (LiAlSi 2 O 6 ), Li has sixfold coordination, but the bond critical points associated to the two longest bonds are characterized by very low electron density values. Similar to what previously found in spodumene and diopside, OÁÁÁO interactions were detected from the topological analysis of q(r), and indicate a cooperative interaction among the lone pairs of neighbouring oxygen atoms. In particular, this kind of interaction has been obtained for the O1ÁÁÁO1 edge shared between two Ga octahedra. Integration over the atomic basins gives net charges of –1.39(10), 2.82(10), 1.91(10) and 0.82(8) e for O (averaged), Si, Ga and Li atoms, respectively. Periodic Hartree–Fock and DFT cal- culations confirm the results obtained by multipole refine- ment of the experimental data. Moreover, the theoretical topological properties of the electron density distribution on the Si 2 O 6 group are very similar to those calculated for spodumene. Keywords Clinopyroxene Á Multipole refinement Á First-principles calculations Introduction Synthetic Ga clinopyroxenes (cpx) are interesting coun- terparts of several natural occurring Al-cpx like spodumene (LiAlSi 2 O 6 ) and jadeite (NaAlSi 2 O 6 ). The substitution of Al with a larger cation is known to provoke significant changes in the geometry of these structures, but the presence of Ga induces other intriguing effects that have yet to be explained. In the cpx structure, the SiO 4 groups form chains of tetrahedra (T) elongating along [001], which are linked to strips where octahedral sites (M1) alternate with M2 sites with maximum coordination number of 8. Changes in the population of the M1 and M2 sites imply rotation of the basal plane of the tetrahedron around [100], yielding stretching or shrinking of the chain along [001]. For instance, in spodumene and jadeite, which have the same M1 cation (Al), but either a small 6-fold coordinated cation (Li) or a large (6 + 2)–coordinated cation (Na) at the M2 site, respectively, the chain of tetrahedra has opposite rotation values with respect to the completely extended conformation, and is thus classified as S- and O-rotated Electronic supplementary material The online version of this article (doi:10.1007/s00269-007-0167-5) contains supplementary material, which is available to authorized users. R. Bianchi Á A. Forni (&) CNR-Istituto di Scienze e Tecnologie Molecolari, via Golgi 19, 20133 Milan, Italy e-mail: alessandra.forni@istm.cnr.it F. Ca ´mara Á R. Oberti CNR-Istituto di Geoscienze e Georisorse (Sezione di Pavia), via Ferrata 1, 27100 Pavia, Italy H. Ohashi HASHI Institute for Silicate Science Nishinakanobu, 1-9-25 Shinagawa, Tokyo 142-0054, Japan 123 Phys Chem Minerals (2007) 34:519–527 DOI 10.1007/s00269-007-0167-5