Petrography of plagioclase

There are two types of plagioclase contained in this sample. The first type is zoned plagioclase feldspar, which changes in composition from being rich in calcium in the core to being rich in sodium towards the rim. These zoned plagioclase feldspars form as large phenocrysts. The second type of plagioclase crystals are smaller grains contained within altered amphibole which are not zoned. The large grains of plagioclase have well faceted grain boundaries and some are even twinned with other plagioclase grains. An example of the twinning in the plagioclase crystals is the bottom picture. Both of these pictures have been taken under crossed polars and the field of view it 2 mm accross. The top picture is a plagioclase crystal which contains inclusions along old growth surfaces. These rings would be analogous to growths rings in a tree.