Clouded-untwinned albite in the Siljan granite, central Sweden.

Untwinned, chemically pure albites clouded by tiny inclusions are common in the Siljan granite, one of two granites in the Siljan meteorite impact crater. The twinned plagioclases (albite and oligoclase) of the granite have been sericitized and chloritized, whereas the microcline has been partially or completely pseudomorphed by the untwinned albite (Ab_99.5An_0.3Or_0.2). This albitization occurred during low-grade metamorphism of the granite. This also resulted in the formation of epidote, sphene and actinolite as well as the chloritization and/or phengitization of biotite, chloritization of hornblende and decomposition of magnetite and Fe-Ti oxides. The inclusions in the untwinned albite are mostly Fe and Ti oxides derived from alteration of biotite, magnetite and Fe-Ti oxides.-R.A.H.