American Mineralogist, Volume 95, pages 832–838, 2010 0003-004X/10/0506–832$05.00/DOI: 10.2138/am.2010.3396 832 High-pressure behavior of Ca/Na clinopyroxenes: The effect of divalent and trivalent 3d-transition elements FABRIZIO NESTOLA, 1,2, * TIZIANA BOFFA BALLARAN, 3 ROSS J. ANGEL, 4 JING ZHAO, 4 AND HARUO OHASHI 5 1 Dipartimento di Geoscienze, Università di Padova, Via Giotto 1, I-35137 Padova, Italy 2 Istituto di Geoscienze e Georisorse, CNR-Padova, Via Giotto 1, I-35137 Padova, Italy 3 Bayerisches Geoinstitut, Universität Bayreuth, Universitätstrasse 37, D-95448 Bayreuth, Germany 4 Crystallography Laboratory, Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, U.S.A. 5 Hashi Institute for Silicate Science, Nishinakanobu 1-9-25, Shinagawa, Tokyo 142-0054, Japan ABSTRACT Two C2/c pyroxenes, CaMnSi 2 O 6 and CaZnSi 2 O 6 , were investigated at high pressure by single- crystal X-ray diffraction. Their unit-cell parameters were determined at 9 different pressures up to 7.55 and 8.23 GPa, respectively. No evidence of phase transformations was found for either sample throughout the investigated pressure range. The pressure-volume data were fit using a third-order Birch-Murnaghan equation of state and it was possible to refine simultaneously the unit-cell volume V 0 , the bulk modulus K T0 and its first pressure derivative K′ providing the following coefficients: V 0 = 462.01(3) Å 3 , K T0 = 97.3(4) GPa, K′ = 7.5(2) for CaMnSi 2 O 6 and V 0 = 444.79(2) Å 3 , K T0 = 109.6(5) GPa, K′ = 7.0(2) for CaZnSi 2 O 6 . The substitution of Zn for Mn at the M1 crystallographic site causes a strong decrease of the bulk modulus (by about 11%). Intensity data were collected at several pressures for both samples. The main compression mechanism is related to the tetrahedral chain kinking and the consequent shortening of the Ca-O3 long bond distances, which show a markedly larger decrease for the CaMnSi 2 O 6 sample. Comparing the results obtained in this work with previous data for Ca/ Na-bearing C2/c pyroxenes containing trivalent and divalent 3d-transition elements at M1 site, we identified two distinct compressibility trends: for a similar unit-cell volume at room pressure the pyroxenes with Na on M2 and trivalent cations on M1 are more compressible than those having Ca on M2 and divalent cations at the M1 site. The analysis of the structure evolution for all samples indicates that this is strongly correlated with the evolution of kinking of the tetrahedral chains and the shortening of the M2-O3 bonds. Keywords: Pyroxenes, transition elements, high pressure, diffraction INTRODUCTION Previous investigations of low temperature or of high pres- sure on C2/c Na and Ca pyroxenes containing trivalent and divalent 3d-transition elements Fe 3+ , V 3+ , Ti 3+ , Cr 3+ , Fe 2+ , and Ni 2+ have been performed to characterize their influence on the thermodynamic properties of compressibility and thermal expan- sion (Redhammer et al. 2003; Nestola et al. 2005, 2006, 2008a, 2008b; Ullrich et al. 2009, 2010). For CaM1Si 2 O 6 compounds, it has been recently suggested that their elastic properties (e.g., bulk modulus K T0 ) can be influenced by the presence of a 3d- transition element at the M1 site resulting in a greater stiffness with respect to the CaM1Si 2 O 6 with no 3d-transition elements at M1 site (Nestola et al. 2005). Such an observation excluded a simple linear inverse relationship between bulk modulus, K T0 , and unit-cell volume, as proposed for many isostructural series (Anderson and Anderson 1970 and references therein). In the last couple of years, high-pressure investigations also have been performed on NaM1Si 2 O 6 silicates with M1 = Fe 3+ , V 3+ , Ti 3+ , and Cr 3+ (Nestola et al. 2006; Ullrich et al. 2009, 2010; Boffa Ballaran et al. 2009) with the identical experimental procedures used for CaNiSi 2 O 6 (Nestola et al. 2005) and CaFeSi 2 O 6 (Nestola et al. 2008). In this study, we present the high-pressure behavior of two additional end-member compositions, CaMnSi 2 O 6 and CaZnSi 2 O 6 [e.g., relative crystal structures described by Ohashi (2003)] to provide more reliable constraints on the response to pressure of Ca and Na silicate pyroxenes having divalent and trivalent 3d-transition elements at the M1 site. Such an investiga- tion provides new information on the effect that minor elements could have on thermodynamic properties (e.g., compressibility) of an important upper-mantle family like clinopyroxenes. It is not rare that natural clinopyroxenes can host minor elements such as the 3d-transition ones (e.g., Ti, V, Cr, Mn, Zn), and the aim of this work is to provide new data on their effect on compress- ibility and structural deformation mechanisms. EXPERIMENTAL METHODS The CaZnSi 2 O 6 crystal studied in this work was chosen from the synthesis run described in Ohashi et al. (1996). This sample was synthesized by solid-state reaction at 6 GPa and 1470 K for 20 h using a belt-type high-pressure apparatus and a mixture of crystalline Ca 2 ZnSi 2 O 7 , ZnO, and 2SiO 2 . The CaMnSi 2 O 6 crystal was selected from the synthesis run obtained by Ohashi (2003). They used a link- * E-mail: fabrizio.nestola@unipd.it