Exsolution phenomena in pyroxenes from the Balmuccia Massif, NW-Italy
Skrotzki, Werner; Müller, Wolfgang F.; Weber, Klaus
published: Feb 21, 1991
manuscript accepted: Jul 20, 1990
manuscript received: Mar 14, 1990
ArtNo. ESP147050301001, Price: 29.00 €
Abstract Pigeonite, orthopyroxene and clinoamphibole exsolution lamellae in slowly cooled augite are studied by conventional and high resolution transmission electron microscopy. The pigeonite lamellae lie close to the (100) and (001) planes of augite with about zero (generation I) and about 15° deviation (generation II). With increasing width of the lamellae, the interphase boundary becomes semicoherent. From a certain thickness, the pigeonite I lamellae are seen to contain (100) stacking faults and antiphase boundaries. The nucleation of pigeonite II occurs preferentially at dislocations. Semicoherent orthopyroxene lamellae lie parallel to the (100) plane of augite. They contain a high density of clinoenstatite lamellae on (100) planes. Clinoamphibole lamellae lie parallel to the (010) plane of augite. Their formation is via stacking faults, with a 1/2  displacement vector, which are bounded by partial dislocations with Burgers vectors of the same type. The minimum width of the lamellae is 9Å, i.e. one double chain. Broadening of the lamellae is by the motion of partial dislocations along the interface. With increasing thickness, the clinoamphibole lamellae become semicoherent. Nucleation of clinoamphibole also occurs at augite/orthopyroxene interfaces. The pigeonite/augite interface orientations and relative lattice rotations are used as geothermometers. Exsolution temperature estimates are about 900°C and ≤ 500°C for pigeonite generations I and II, respectively. These results, as well as the intergrowth relations between different phases, suggest the exsolution sequence: Orthopyroxene, pigeonite I, clinoamphibole, pigeonite II. The exsolution phenomena are related to the tectonic evolution of the Balmuccia Massif, as established previously from a microstructural study of the peridotites.