A TEM Study of Ca-rich Orthopyroxenes with Exsolution Products

Fernando Cámara (camara@crystal.unipv.it)¹, Michele Zema (michele@elicona.unipv.it)², Jean-Claude Doukhan (jean-claude.doukhan @univ-lille1.fr)³ & M. Chiara Domeneghetti (domeneghetti@crystal.unipv.it)⁴

¹ Centro di Studio per la Cristalochimica e la Cristallografia, via Ferrata 1, 27100 Pavia, Italy

² Centro Grandi Strumenti, via Bassi 21, 27100 Pavia, Italy

³ Laboratoire de Structure et Propriétés de l'Etat Solide, Université des Sciences et Technologies de Lille, Bat C6, 59655 Villeneuve d'Ascq Cedex, France

⁴ Diparimento di Scienze della Terra, via Ferrata 1, 27100 Pavia, Italy

Anomalous kinetics for disordering Fe-Mg exchange reaction in Ca-rich orthopyroxene from the Johnstown meteorite (diogenite) has been found by different authors (Zema et al., 1997, 1999a; Heinemann et al., 1999). This orthopyroxene contains exsolution products that were supposed to be responsible for the low values of the disordering rate constants and the high activation energy measured for this sample. The application of these kinetic constants would lead to abnormal cooling rates without physical meaning. Two orthopyroxene single crystals from Johnstown diogenite were studied by several transmission electron microscopy techniques (TEM, ATEM, HREM, LACBED) in order to elucidate their exsolution products, types, topology, chemistry and their probable relationships with kinetics. One of the crystals was previously annealed at 750ºC (Zema et al., 1999b). For comparison this study also included a terrestrial granulitic orthopyroxene with known "normal" disordering kinetics, and a Ca-rich orthopyroxene of volcanic origin which was supposed to be free of exsolution products. TEM results showed that all the studied samples contain very fine clinopyroxene exsolution lamellae (several unit-cells wide), parallel to (100) of orthopyroxene, with variable volume frequency. High-resolution electron images and nanoprobe analyses on lamellae show that they are C2/c augitic clinopyroxene. Additionally, only Johnstown orthopyroxenes contain one unit-cell wide Guinier-Preston zones (GPZ). These represent a chemically distinct pyroxene (Ca-rich) and their distribution in Johnstown is pervasive. They are disk shaped (ca. 100 nm diameter) and oriented parallel to (100) of host orthopyroxene, spaced every ca. 20 nm along [100], and separated from clinopyroxene lamellae by precipitate free zones. GPZs are still present after annealing experiments at the kinetic studies temperatures. Large Angle Convergent Beam Electron Diffraction shows that GPZ produces rotations of Bragg lines of ca. 1 mrad. The high density of these defects is proposed to be responsible for the anomalous kinetic behavior.

Heinemann R, Fischer A, Lueder T & Kroll H, Terra Abstract, 11, 648, (1999).

Zema M, Domeneghetti MC & Tazzoli V, Terra Abstract, 9, 442, (1997).

Zema M, Domeneghetti MC & Tazzoli V, Am. Mineralogist, 84, 1895-1901, (1999a).

Zema M, Domeneghetti MC & Tazzoli V, Phase Transitions, 69, 35-46, (1999b).

Incongruent Melting of Ferromagnesian Cordierite Studied by ATEM

Gian Carlo Capitani (gian-carlo.capitani @univ-lille1.fr) & Jean Claude Doukhan (jean-claude.doukhan@univ-lille1.fr)

Laboratoire des Structure et Propriétés de l'Etat Solide, Université des Sciences et Technologies de Lille, 59655 - Villeneuve d'Ascq CEDEX, France

The importance of cordierite in felsic igneous rocks is well known (e.g. Clarke, 1995). However, excepted the work on pure end members Mg₂Al₄Si₅O₁₈ (Schreyer and Schairer, 1961) and Fe₂Al₄Si₅O₁₈ (Schairer and Yagi, 1952), there have been quite few studies on the melting of cordierite. Data are also scare as regard the microstructures developed in the mineral as the incongruent melting progresses. We report an analytical electron microscopy study of a natural cordierite with composition Mg_1.3Fe_0.7Al₄Si₅O₁₈ annealed at various temperatures up to values close to the liquidus temperature. Annealings were performed at room pressure and under controlled oxygen fugacity. Partial melting is observed in samples annealed at 900°C in the form of ribbons of bubbles of Si-rich glass containing very tiny Fe-rich crystallites (Fig. a). This local melting is assumed to be induced by the presence of foreign H₂O-rich phases, in agreement with the observations on the untreated sample. Samples annealed at 1350°C show premelting microstructures in the form of elongated glassy veins parallel to simple crystallographic directions (Fig. b).

In some areas the amount of melt is more substantial. The following reaction occurred: cordierite ­> mullite + spinel + silica-rich glass. At 1430°C spinel is no longer present and the incongruent melting is governed by the reaction: Fe-cordierite ­> Mg-cordierite + mullite + Fe-silica-rich glass. It thus appears that melting of the solid solution cordierite can be quite complex with different solid phases occurring as function of the (T, P) conditions and on the initial composition of the mineral. More detailed studies are in progress on samples with different Mg:Fe ratios.

Clarke DB, Mineral. Mag., 59, 311-325, (1995).

Schreyer W and Schairer JF, Journal of Petrology, 2, 324-406, (1961).

Schairer JF & Yagi K, Am. J. Science, Bowen volume, 471-512, (1952).