Phonolite #21

Description

This specimen is a light gray, earthy-looking phonolite, an uncommon kind of extrusive igneous rock. Although difficult to see in hand specimen, it contains feldspars (anorthoclase phenocrysts and orthoclase in the matrix), nepheline, and pyroxene (aegirine) as major minerals. This sample is from Beacon Hill, Cripple Creek, Colorado, USA. This is 10 cm across.