File:Cracked geode (Las Choyas Geode Deposit, near-latest Eocene, ~35 Ma; Chihuahua, Mexico) (31911142424).jpg

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Geode from the Eocene of Mexico. (47th Annual Gem, Mineral, Fossil & Jewelry Show of Greater Cincinnati, Ohio, USA)

Geodes are small to large, subspherical to irregularly-shaped, crystal-lined cavities in rocks. They form when water enters a void in a host rock and precipitates crystals. The most common geode-lining mineral is quartz.

This specimen is from 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 Cracked geode (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@N05/31911142424 (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
current02:03, 6 December 2019Thumbnail for version as of 02:03, 6 December 20192,991 × 1,650 (3.51 MB)Ser Amantio di Nicolao (talk | contribs)Transferred from Flickr via #flickr2commons

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