Petrography of a dome rock sample

(field of view = 2 mm)

Click to view: Zoned plagioclase crystal under crossed polars.

The dome sample studied is fine-grained and contains visible crystals of plagioclase, amphibole, and pyroxene. These phenocrysts, as they are known, make up approximately 40% of the rock and range in size from 1 - 4mm. The finer grained material, or groundmass, makes up 60% of the rock and is less than 0.5mm. This groundmass consists of small grains of plagioclase, quartz, pyroxene, and oxides. Included are some medium sized grains of carbonate throughout the sample which have rough edges. The amphibole grains are strongly altered and poikilitically enclose smaller grains of unzoned plagioclase. Poikilitic means that a single larger grain of amphibole completely surrounds smaller grains of plagioclase. The amphibole grains are rough around the edges. Some of the grains contain unaltered amphibole, though most are strongly altered. The pyroxenes range in size throughout the sample and have well defined grain boundaries. There are also very rare grains of olivine that are only visible by microscope, not in hand sample. Quartz also appears where feldspars have been partially remelted.