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The thermal behavior of richterite

Mario Tribaudino^1,*, Marco Bruno², Gianluca Iezzi^3,4,, Giancarlo Della Ventura⁵ and Irene Margiolaki⁶

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 Geofisica e Vulcanologia, Via di Vigna Murata 605, I-00143 Roma, Italy
⁵ Dipartimento di Scienze Geologiche, Universita 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

Correspondence: ^* E-mail: mario.tribaudino{at}unipr.it

The thermal behavior of synthetic richterite ^ANa^B(NaCa)^CMg₅^TSi₈O₂₂(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 _s are 410(3), 215(2), and 300(2) K for a, b, and asinβ, respectively. The axial expansion pattern below room temperature is _b> _c> _a, whereas above room T it is _b> _a> _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 V₀ = 908.20 (0.04) ų, k = 1 (4), Q₀ =32 (2) MJ, and the Debye temperature _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 _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 deformation of pyroxene.

Key Words: Richterite • amphibole • thermal expansion • Debye model • comparison with pyroxenes