Agate-filled geode from the Eocene of Mexico. (Jeff Smith collection)
This agate nodule 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.
Tags: Las Choyas geode deposit Eocene Chihuahua Mexico Tertiary agate nodule nodules quartz geodes
Geode from the Eocene of Mexico.
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.
Tags: geode Las Choyas Geodes Deposit Chihuahua Mexico Eocene
Agate-filled geode from the Eocene of Mexico. (Jeff Smith collection)
This agate nodule 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.
Tags: Las Choyas geode deposit Eocene Chihuahua Mexico Tertiary agate nodule nodules quartz geodes
Agate-filled geode from the Eocene of Mexico. (Jeff Smith collection)
This agate nodule 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.
Tags: Las Choyas geode deposit Eocene Chihuahua Mexico Tertiary agate nodule nodules quartz geodes
Faulted geode from the Eocene of Mexico. (exhibit photo)
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 remarkable faulted geode 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.
Tags: faulted geode Las Choyas Geodes Deposit Chihuahua Mexico Eocene fault faults