Dacite: R1826

Item

Title
Dacite: R1826
Description
Collection: Lafayette College
Sample number: R1826 (Ward's Natural Science Establishment, Inc.)
Location: N.W. of Helena, Montana
Rock: Dacite
Prominent minerals: Contains plagioclase, biotite, amphiboles, pyroxenes, quartz.
Type: Felsic/intermediate extrusive igneous rock.
Description of hand sample: Porphyritic rock containing gray, white and black grains. White grains appear to be larger than the others, and gray grains mostly make up the matrix. Dark grains are small and not as abundant.
Description of thin section: A large amount of glass is present in the rock, indicated by total extinction in XPL. Plagioclase feldspar is also abundant throughout, as indicated by twinning and low birefringence. Small biotite grains are brown and display slight color changes as the stage is rotated. A small amount of pyroxenes and amphiboles are present, indicated by their characteristic cleavage. Some quartz is present, indicated by low birefringence and undulose extinction.
Potential history: Dacite is an extrusive felsic/intermediate rock. The rock photographed likely resulted as a silicic magma started to cool gradually (forming the phenocrysts) before being rapidly exploded leading to the creation of a fine matrix with glassy components.
Source
Wards Collection of North American Rocks
Identifier
R1826
Mediator
Tamara Carley
Rights Holder
Lafayette College Department of Geology and Environmental Studies
Creator
Devon Leach, 3178
Type
Igneous
Media
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