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Exsolution phenomena in pyroxenes from the Balmuccia Massif, NW-Italy

Werner Skrotzki, Wolfgang F. Mueller and Klaus Weber
Exsolution phenomena in pyroxenes from the Balmuccia Massif, NW-Italy
European Journal of Mineralogy (February 1991) 3 (1): 39-61


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 degrees 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 [101] displacement vector, which are bounded by partial dislocations with Burgers vectors of the same type. The minimum width of the lamellae is 9A, 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 degrees C and < or = 500 degrees 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.

ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 3
Serial Issue: 1
Title: Exsolution phenomena in pyroxenes from the Balmuccia Massif, NW-Italy
Affiliation: Univ. Goettingen, Inst. Geol. Dynam. Lithosphaer., Gottingen, Federal Republic of Germany
Pages: 39-61
Published: 199102
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 3 p.
Accession Number: 1994-052605
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
N44°04'60" - N46°28'60", E06°37'60" - E09°10'60"
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from Geoline, Bundesanstalt fur Geowissenschaften und Rohstoffe, Hanover, Germany
Update Code: 199424
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