New types of twinning in enstatite and pseudobrookite from Lemolo Lake, Douglas County, Oregon, USA

摘要

Enstatite crystals twinned on {031} have been found in a basaltic andesite at Lemolo Lake, Douglas County, Oregon. Multiple twins about a center have been observed. Pseudobrookite twins are present in the vesicles of the same rock. The composi-tion planes observed for psuedobrookite are {101}, {110), {430}, {210} and {310}. A {031} twin is also reported from the Big Lue Mountains, Arizona, and an unidentified twin from the Anakies, Victoria, Australia. Catalyzed filamentary growth is suggested as a possible explanation for the existence of so many twin planes.