American Mineralogist, Volume 93, pages 1659–1665, 2008 0003-004X/08/0010–1659$05.00/DOI: 10.2138/am.2008.2895 1659 The thermal behavior of richterite Mario Tribaudino, 1, * Marco bruno, 2 Gianluca iezzi, 3,4, † Giancarlo della VenTura, 5 and irene MarGiolaki 6 1 Dipartimento di Scienze della Terra, Università di Parma, Viale G.P. Usberti 157/A, I-43100, Parma, Italy 2 Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, Via Valperga Caluso, 35, I-10123, Torino, Italy 3 Dipartimento Scienze della Terra, Università G.d’Annunzio, I-66013 Chieti, Italy 4 Department of Seismology and Tectonophysics, Istituto Nazionale di Geoisica e Vulcanologia, Via di Vigna Murata 605, I-00143 Roma, Italy 5 Dipartimento di Scienze Geologiche, Università di Roma Tre, Largo San Leonardo Murialdo 1, I-00146 Roma, Italy 6 European Synchrotron Radiation Facility (ESRF), B.P. 220, F-38043, Grenoble, France absTracT The thermal behavior of synthetic richterite A Na B (NaCa) C Mg 5 T Si 8 O 22 (OH) 2 , crystallized at 800 °C and 0.35 GPa under hydrothermal conditions, was studied by serial powder diffraction experiments using synchrotron radiation between 123 and 873 K. The a, b, and β cell parameters show a non-linear behavior, whereas the c parameter shows a linear trend; values of the saturation temperature θ s are 410(3), 215(2), and 300(2) K for a, b, and asinβ, respectively. The axial expansion pattern below room temperature is α b > α c > α a , whereas above room T it is α b > α a > α c , the difference being related to the different saturation temperatures of the individual cell parameters. The thermal expansion was modeled following the Debye approximation for the density-of-state of phonons; the refined parameters are V 0 = 908.20 (0.04) Å 3 , k = 1 (4), Q 0 =32 (2) MJ, and the Debye temperature θ D = 586(31) K. The non-linear behavior at low T is well described without systematic differences between the data and the model. The volume thermal expansion coefficient α V changes significantly with temperature, also at temperatures higher than room temperature. It is suggested that this may occur also in other amphiboles and pyroxenes, requiring critical re-examination of the available data. A comparison of the strain tensor for the thermal expansion between amphiboles and pyroxenes shows that in amphiboles the major deformation occurs onto the (010) plane, whereas in pyroxenes it occurs along the b axis. Moreover, the major deformation on (010) in Ca-bearing pyroxenes occurs along the bond of the M2 cation with the furthermost O3 atoms, whereas in compositionally related Ca-amphiboles (i.e., tremolite) it occurs in a direction rotated by 10–20° to a*, i.e., in a direction not corresponding to that of the M4-O5 bonds. It is proposed that the M4 polyhedron contributes less to the thermal deformation of amphibole than the M2 polyhedron contributes to the thermal deforma- tion of pyroxene. Keywords: Richterite, amphibole, thermal expansion, Debye model, comparison with pyroxenes * E-mail: mario.tribaudino@unipr.it † Present address: Dipartimento di Geotecnologie per l’Ambiente ed il Territorio, Università G.d’Annunzio, I-66013 Chieti, Italy. inTroducTion Richterite is a C2/m amphibole, with the general formula A (Na,K) B (Na,Ca) C Mg 5 T Si 8 O 22 (OH) 2 . Its composition is similar to that of tremolite [ A  B Ca 2 C Mg 5 T Si 8 O 22 (OH) 2 ], but with the A site almost completely filled by Na and K: the entry of a mon- ovalent cation into the A site is compensated by the coupled substitution of one Ca atom per formula unit (apfu) with Na in the M4 (B) site. The behavior of richterite at high temperature and high pres- sure is of interest in geophysical modeling because richterite and especially K-richterite is stable over a relatively large T-P range. K-richterite, due to its high-pressure stability conditions (up to 14 GPa, Trønnes 2002) is considered, together with other hydrated minerals, as a possible water carrier phase in the deep zone of a subducted plate (e.g., Inoue et al. 1998; Frost 2007). Therefore the crystal-chemical behavior of this important rock- forming mineral under non-ambient conditions is of interest in developing accurate thermodynamic models. Despite the relevance of richterite as a rock-forming mineral, in situ high-temperature (high-T) data are scarce (Welch et al. 2007), as well as for other amphiboles. Cameron et al. (1983) investigated the evolution of the crystal structure as a function of T and determined the thermal expansion of synthetic A K and A Na fluororichterites by linear interpolation of few data. Linear inter- polation and, in general, empirical fitting of thermal-expansion data (Pawley et al. 1996; Holland and Powell 1998), may pro- vide reasonably approximated thermal expansion coefficients to be used in thermodynamic calculations of phase equilibria. However, with this procedure, the physical information on