File:Sandstone-phonolite intrusive contact (Buck Ranch, Highwood Mountains, Montana, USA) 1 (49060639223).jpg

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Dark-colored rock at top = porphyritic phonolite Light-colored rock at bottom = sandstone

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.

This specimen shows the intrusive contact between the phonolite and the country rock - in this case, the Cretaceous-aged Eagle Sandstone.

Geologic units & age: Shonkin Sag Laccolith, Ypresian Stage, Early Eocene, 50-53 Ma & Eagle Sandstone, Upper Cretaceous

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 Sandstone-phonolite intrusive contact (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@N05/49060639223. 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 20193,996 × 1,929 (4.91 MB)Ser Amantio di Nicolao (talk | contribs)Transferred from Flickr via #flickr2commons

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