ORIGINAL PAPER Nadia Curetti Æ Davide Levy Æ Alessandro Pavese Gabriella Ivaldi Elastic properties and stability of coexisting 3T and 2M 1 phengite polytypes Received: 11 July 2005 / Accepted: 12 September 2005 / Published online: 6 December 2005 Ó Springer-Verlag 2005 Abstract The elastic properties of coexisting natural 3T and 2M 1 phengite samples (Cima Pal, Sesia Zone; Val Savenca; Western Alps, Italy) with similar chemical compositions have been studied by room temperature– high pressure powder diffraction, using synchrotron radiation on the ID9A beam-line at ESRF (Grenoble, France). The P–V curves have been modelled by the Birch–Murnaghan model; a third-order expansion fitted to the experimental data yields for 3T and 2M 1 K 0 = 60.4(±0.7) GPa, K¢=5.79(±0.11) at V 0 = 703.8851 A ˚ 3 , and K 0 =57.3(±1.0) GPa, K¢=6.97(± 0.24) at V 0 = 938.8815 A ˚ 3 , respectively. The relative stability of 3T vs. 2M 1 has been explored as a function of pressure and temperature in terms of configuration and deformation contributions to the Gibbs energy, using the elastic properties determined here and other thermodynamic parameters from earlier investigations. The results pre- sented agree with the hypothesis of stability of the 3T polytype in the high pressure regime. Keywords 2M 1 –3T phengites Æ P–V EoS Æ Relative stability Introduction Micas are widespread rock-forming minerals involved in a variety of petrogenetic reactions. In particular, phengites [KAl 2–y (Fe,Mg) y (Si 3+y Al 1–y )O 10 (OH) 2 , ide- ally y=0.5], which are dioctahedral micas of the muscovite–celadonite series (nomenclature after Rieder et al. 1999), commonly occur in metamorphic rocks and have been used as markers to estimate pressure (P) and temperature (T) of crystallisation. Sassi et al. (1994) argued on the ground of occurrence that the stability of the 2M 1 (S.G. C2/c) and 3T (S.G. P3 1 12) polytypes is dependent on the P/T ratio of formation: large P/T values are supposed to promote crystallisation of 3T phengite, presumably because of 3T having a more flexible structure than 2M 1 to arrange cations in the octahedral sheet. In fact, the trigonal polytype provides two symmetry-independent octahedral sites (M2 and M3), against only one in 2M 1 (Ferraris et al. 1995; Ivaldi et al. 2001a, b; Ferraris and Ivaldi 2002). All this, in turn, relates to the occurrence of cation order, which Pavese et al. (1997, 1999a, 2001, 2003) have extensively investigated by neutron powder diffraction. The struc- tural configuration of hydroxyl, too, is believed to play a role for stabilisation of a given polytype (Ferraris et al. 1995). However, the stability of 3T vs. 2M 1 phengites is still an open question, and the difficulty to find natural samples bearing coexisting trigonal and monoclinic good quality crystals has hitherto hindered the possi- bility to carry out a reliable comparison between the thermo-elastic properties of these two polytypes. Note that, in general, dioctahedral micas have been to date subject to relatively few studies aimed at investigating their high pressure and temperature behaviour, as re- viewed by Zanazzi and Pavese (2002). Smyth et al. (2000) report the results of a room temperature high pressure X-ray single crystal study of synthetic 3T and 2M 1 polytypes, paying special attention to their struc- tural response as a function of P, but still leaving uncertainties as to their P–V equations of state (EoS). In the present case, we have at our disposal an interesting rock sample from Cima Pal (Sesia Zone; Val Savenca; Western Alps, Italy) exhibiting occurrence of both 3T and 2M 1 phengite polytypes, with similar N. Curetti Æ D. Levy Æ G. Ivaldi Dipartimento Scienze Mineralogiche e Petrologiche, Universita` degli Studi di Torino, Via Valperga Caluso 35, 10125 Turin, Italy A. Pavese (&) Consiglio Nazionale delle Ricerche, IDPA, Sezione di Milano, Via Botticelli 23, Milan, Italy A. Pavese Dipartimento Scienze della Terra, Sezione Mineralogia, Universita` degli Studi di Milano, Via Botticelli 23, 20133 Milan, Italy E-mail: alessandro.pavese@unimi.it Tel.: +39-2-50315603 Fax: +39-2-50315597 Phys Chem Minerals (2006) 32: 670–678 DOI 10.1007/s00269-005-0044-z