File:Geodes and agate-filled nodules (Las Choyas Geode Deposit, near-latest Eocene, ~35 Ma; Chihuahua, Mexico) (32713526596).jpg
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[edit]DescriptionGeodes and agate-filled nodules (Las Choyas Geode Deposit, near-latest Eocene, ~35 Ma; Chihuahua, Mexico) (32713526596).jpg |
Geodes and agate-filled nodules mined from the Eocene of Mexico. (public exhibit photo by Jeff Smith; shot taken in the field on 29 October 2004) This is Mexico's famous Las Choyas Geode Deposit. At this locality, geodes occur in structurally-folded, rhyolitic volcanic tuffs (ash flow tuffs) of Middle to Late Eocene age (~35 to 44 Ma). The geodes were originally cavities in the rhyolitic rock. These cavities (lithophysae) formed before the rock completely lithified. The original ash flow deposit had some subspherical structures known as spherulites, composed of glassy to cryptocrystalline material (many felsic extrusive igneous rocks have these). Expanding gases in the spherulites destroyed the material, resulting in empty spaces. In the near-latest Eocene (~35 Ma), regional rhyolite dome intrusions resulted in hot groundwater percolating through the rocks, leaching out silica and precipitating quartz in the lithophysae/cavities. About eighty percent of the geodes mined at this site are solid agate/quartz nodules. Locality: Las Choyas Geode Deposit, northern Aldama County, north-central Chihuahua State, northern Mexico Info. synthesized from: Keller (1977) - Quartz geodes from near the Sierra Gallego area, Chihuahua, Mexico. Mineralogical Record 10: 207-212. Smith (2010) - The Las Choyas Geode Deposit, Chihuahua, Mexico. Rocks & Minerals 85: 112-122. |
Date | |
Source | Geodes and agate-filled nodules (Las Choyas Geode Deposit, near-latest Eocene, ~35 Ma; Chihuahua, Mexico) |
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@N05/32713526596 (archive). It was reviewed on 10 October 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
10 October 2019
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current | 07:12, 10 October 2019 | 1,819 × 1,392 (2.03 MB) | Ser Amantio di Nicolao (talk | contribs) | Transferred from Flickr via #flickr2commons |
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File change date and time | 18:54, 6 February 2017 |
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Date and time of digitizing | 14:06, 1 May 2010 |
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Date metadata was last modified | 13:54, 6 February 2017 |