File:Porphyritic phonolite (Shonkin Sag Laccolith, Early Eocene, 50-53 Ma; Buck Ranch, Highwood Mountains, Montana, USA) 1 (49061149591).jpg

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Porphyritic phonolite from the Eocene of Montana, USA.

Phonolite is a highly alkaline, intermediate to mafic, finely-crystalline, extrusive igneous rock - it is a scarce type of volcanic lava. This sample is a porphyritic phonolite and is intrusive in origin - it's from a laccolith in Montana. Its phenocrysts include salite pyroxene (a type of diopside), pseudoleucite, olivine, phlogopite mica, and apatite. The finely-crystalline groundmass is composed of pseudoleucite, olivine, salite, sanidine, biotite, titanomagnetite, apatite, zeolites, and minor carbonate.

Geologic unit & age: Shonkin Sag Laccolith, Ypresian Stage, Early Eocene, 50-53 Ma

Locality: Buck Ranch, ~3.0 miles south-southwest of Geraldine, northeastern flank of the Highwood Mountains, southern Choteau County, northern Montana, USA
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Source Porphyritic phonolite (Shonkin Sag Laccolith, Early Eocene, 50-53 Ma; Buck Ranch, Highwood Mountains, Montana, USA) 1
Author James St. John

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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/49061149591 (archive). It was reviewed on 6 December 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0.

6 December 2019

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Date/TimeThumbnailDimensionsUserComment
current02:05, 6 December 2019Thumbnail for version as of 02:05, 6 December 20193,295 × 2,313 (5.33 MB)Ser Amantio di Nicolao (talk | contribs)Transferred from Flickr via #flickr2commons

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