Complex exsolution microstructures in inverted pigeonites from the Sjelset Igneous Complex, Rogaland, SW Norway

An optical and electron microscope study has been made of inverted pigeonite from the Sjclset Igneous Complex, Rogaland, SW Norway. The inverted pigeonites reveal a Stillwater-type of microstructure: the pigeonite to orthopyroxene transformation has been pre- and post-dated by two stages of augite exsolution (parallel to (001)pig and (1OO)opx, respectively). In addition. two distinct microstructural domains have been recognized: (1) single and (2) clusters of inverted pigeonite crystals. Most clusters of pigeonite crystals have been replaced by single orthopyroxene crystals which exhibit domains of differently oriented (001 )pig exsolution lamellae. The mechanism of exsiolution is interpreted to be heterogeneous nucleation and growth, while the transformation of pigeonite to orthopyroxene is regarded as massive.