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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