Synchrotron radiation XRPD study of non ideality, thermal expansion and Incommensurate-Normal structure phase transition in melilites

Marco Merlini¹, Mauro Gemmi¹, Gilberto Artioli¹

¹Dipartimento di Scienze della Terra "A. Desio", Università degli Studi di Milano

Melilites are a group of minerals which form continuos solid solutions between the end members gehlenite (Ge, Ca₂Al₂SiO₇) akermanite (Ak, Ca₂MgSi₂O₇), soda-melilite (NaMel, NaCaAlSi₂O₇) and Fe-bearing end members (Fe²⁺-gehlenite – Fe³⁺-gehlenite / Fe²⁺-akermanite – Fe³⁺-akermanite). The importance of these minerals is due to the fact that almost pure terms of the series gehlenite-akermanite are among the first silicates which condensed from the solar nebula and they are found in chondritic meteorites. Melilites crystallise also in alkaline magmatic roks, whose origin is restricted to significant geodynamic environments. Therefore a modelling of their thermodynamic parameters is mandatory in order to improve the accuracy of phase equilibria calculations, which could have important implications in petrological studies (i.e. for accurate determination of the thermal history of meteorites, of the temperature of solidification of alkaline roks ...). The thermodynamic calculations involving phase equilibria of melilites are usually performed assuming the ideality of the solid solution (s.s.) [1]. Nevertheless melilite s.s. has not an ideal behaviour (fig. 1). Our results show that the unit cell volume curve in the ge-ak join has a sigmoidal shape, with a negative and a positive excess molar volume close to ge and ak end-members respectively. This is the general behaviour of non equivalent site substitution mentioned by Newton and Wood [2] for binary silicate solid solutions. An other important feature of melilites is the presence of an incommensurate modulated (IC) structure, especially for ak-rich compositions, which transforms into a normal (N) one upon heating [3,4,5]. The temperature of IC-N phase transition is of 80 °C for pure akermanite and it is increased by Fe/Mg substitution and decreased by Al /(Mg+Si) exchange. We performed high temperature (HT) X-ray powder diffraction (XRPD) on terms of Ge-Ak-NaMel on the italian CRG beamline BM08 (GILDA) at ESRF, Grenoble in order to measure reliable thermodynamics data (volume, thermal expansion) for several composition and to investigate the IC-N phase transition for some terms near Ak end-member. The experiments were performed with monochromatic radiation; the samples were contained in quartz glass capillary, heated with a hot gas blower and data were collected with a translating image plate (T.I.P.) [6]. IC and N phases are marked by a strong difference in thermal expansion along the main crystallographic directions. Thermal expansion measurements allowed to investigate the field stability of IC phase near ak composition. The phase transition is characterised by a contraction along c and also a slight negative volumetric expansion (fig. 2-3). The results obtained comprise also accurate thermal expansion measurements and crystal-structure Rietveld refinements for melilites in Ak-Ge and Ak-NaMel joins in the temperature range 270-1300 K.

Fig. 1 - Volume (Å^3) in the join Ge-Ak at 300 K with the characteristic S-shaped form of the volume curve in binary silicate solid solutions.	Fig. 2 - Variation of c lattice parameter value across the phase transition for akermanite measured with T.I.P. detector at GILDA beamline (ESRF)

Fig. 3 - Thermal expansion of akermanite across the IC-N phase transition

[1] Yoneda S. & Grossman L., 1995, Geoc. Cosmoch. Acta, 59, (16), 3413-3444

[2] Newton R.C. & Wood B., 1980, Am. Mineral., 65, 733-745

[3] Seifert et al., 1987, Phys. Chem. Minerals, 14, 26-35

[4] Rothlisberger et al., 1990, Eur. J. Mineral., 2, 585-594

[5] Yang et al., 1997, Phys. Chem. Minerals, 24, 510-519

[6] Meneghini et al., 2001, Journ. Synchrotron. Rad. 8, 1162-1166