Dead Indian Summit Overlook
Chief Joseph Highway, Park County Wyoming
Photo by Rich Nickel
Chief Joseph Highway, Park County Wyoming
Photo by Rich Nickel
LOOK WHAT I FOUND!
This is where we dedicate one special geological "find" and share knowledge and examples of that particular find at our next meeting.
This is where we dedicate one special geological "find" and share knowledge and examples of that particular find at our next meeting.
APRIL 2024
Geodes, Thundereggs, Vugs and Nodules
Agate and quartz lining a geode from Dryhead, Montana (Jerry Schaber collection)
Photo by James St, John, (CC BY 2.0)
Photo by James St, John, (CC BY 2.0)
The terms geode, vug, nodule and thunderegg describe crystalline/cryptocrystalline structures that have formed in open spaces in a host rock like a basalt, rhyolite, rhyolitic tuff or limestone. They may contain many kinds of minerals but some of the most common are various varieties of quartz, both crystalline and cryptocrystalline, as well as calcite, and opal. The terms are used for masses that are generally spherical to subspherical in shape but there are exceptions. These features (geode, vug, nodule, or thunderegg) are referred to by geologists as being secondary which means they were formed/deposited after the host rock was deposited /solidified. Geodes, vugs, nodules and thundereggs share many common features and form by similar processes. The crystals precipitate from hydrothermal (hot/warm) or groundwater (cooler) fluids, which contain dissolved compounds.These crystals form in open spaces found in a host rock. The conditions needed to form these crystals depend on special geochemical environments. There are a number of valid theories about how the crystals specifically form given the particulars of those environments. But it is the beautiful results, not the geochemical processes, that most rockhounds are interested in.
Since geodes, vugs, nodules and thundereggs often form similarly and have may common features, these terms often get confused and misused. Like with many geologic classification systems there are some specimens which seem to lie somewhere in between. Usage of the terms also varies from place to place around the world. No formal classification system currently is in use to help name these feature/objects. Here is the way these terms are applied in common usage.
Since geodes, vugs, nodules and thundereggs often form similarly and have may common features, these terms often get confused and misused. Like with many geologic classification systems there are some specimens which seem to lie somewhere in between. Usage of the terms also varies from place to place around the world. No formal classification system currently is in use to help name these feature/objects. Here is the way these terms are applied in common usage.
Agate and quartz (amethyst) geode from Absaroka Mountains, Wyoming
Photo: Greg L. Jones 
A geode filled with botryoidal chalcedony from the Absaroka Mountains, WY. The sample is back lit by a lamp.
Photo: Greg L. Jones 
Small (1.5 x 1 inch) geode with black and white chalcedony lined with druzy
smokey quartz from Bear Canyon, Pryor Mountains, MT. This agate occurs at the top of the Amsden Formation. Photo Greg L. Jones |
Geode: A geode is a hollow space where minerals have been precipitated. The minerals grow from the inside of a void towards the middle in a concentric growth pattern. This can lead to layers of cryptocrystalline/crystalline minerals within the void. Geodes need a competent external lining that allows them to separate from their host rock. As the host rock weathers and erodes, the geodes stay intact, separate from the host and become circular rocks. Some geodes are found still embedded in the host rock while others are found as individual rounded specimens in gravel beds and soils. The most common mineral to be found in a geode is quartz. Layers of cryptocrystalline quartz such as chalcedony and agate are common. The inside of the geode it is often covered a with a variety of quartz crystals such as amethyst, citrine, smoky quartz or clear quartz. The inside of the void could also be botryoidal, mammalary, or staglititic chalcedony (in other words no crystals).
Calcite and opal are also common minerals in geodes. There are rare, hardly ever found geodes with many other kinds of minerals. A quick scan of the internet found geodes for sale with ankerite, aragonite, dolomite, goethite, gypsum, kaolinite, marcasite, millerite, pyrite, sphalerite, volkovskite, dioptase, smithsonite, and celestine. (The asking price for some of these were 4 and 5 figure amounts for relatively small geodes.) Geodes do occur in the Big Horn basin in limited amounts especially in the Madison, Amsden and Phosphoria formations. Many of these geodes are small. Some chalcedony geodes have been found in the volcanics of the Absaroka Mountains. Some geodes have been reported from Tertiary conglomerates on the western side of the Bighorn Basin. Occasionally geodes are found in gravel beds along river terraces and riverbeds. |
Chert Nodules in The Mississippian Madison Limestone at Bar- ry’s Landing Montana in Bighorn Canyon National Recreation Area
Photo: Greg L. Jones 
Agate Nodule from Dryhead, Montana
Photo:Greg L. Jones 
Opal and Fluorite Nodule containing up to 5% Bertrandite (beryllium sili- cate) from rhyolite flows near Spor Mountain, Utah
Photo: Greg L. Jones 
Vug with aragonite crystals in fossiliferous limestone from the Mississippian Logan Formation, Heath, Ohio
Photo: James St, John, (CC BY 2.0) 
A 3x3’ quartz vug in the Ordovician Crystal Mountain Sandstone, Coleman Quartz Mine, Arkansas
Photo: Arkansas Geological Survey https://www.facebook.com/101741636586168/ photos/a.508541579239503/656097367817256/?type=3&is_lookaside=1 
Thunderegg from Oregon
Photo: Greg L. Jones 
A 5” in diameter agate, chalcedony and jasper nodule, probably found near Lysite Wyoming
Photo; Gregory L. Jones, specimen from the Jim Platt Collection. |
Nodules: Generally, nodules have no space in them, (while geodes do and are “hollow”). Most nodules like geodes are spherical but occasional irregular shaped nodules occur. Just like geodes, nodules have a competent external lining that allows them to separate from their host rock. As the host rock weathers and erodes, the nodules stay in- tact, separate from the host and become spherical rocks. Some nodules form in a similar fashion to geodes but the space in the host rock has been completely filled in by precipitated cryptocrystalline or crystalline minerals. Others may be formed by crystallization of minerals during deposition of the host rock. Other nodules may form by mineral replacement of the host. Like geodes, nodules can be layered with a concentric growth pattern. Some can have multiple minerals in them while others may be just one composition like chert (cryptocrystalline quartz) or hematite (iron nodules). In gravel beds many of the agate, chalcedony, jasper and chert pebbles may have started out as nodules in a host rock. They were eroded from that host and then deposited in the gravel. Like geodes, nodules can have a wide variety of mineral composition but nodules of cryptocrystalline quartz (agate, chalcedony, jasper, chert, etc.) or opal are the most common. Some nodules may have an open space with small, often drusy, crystals in it. Is it a geode? Depends on who you ask. If the opening is small, some collectors and rockhounds will still call it a nodule. Others may call it a geode based on the common usage explained here. Who’s right? Depends on the viewer’s point of view and preference.
In the Bighorn Basin chert, agate, chalcedony and other cryptocrystalline quartz nodules are common in the Bighorn Dolomite, Madison Limestone, Amsden Formation, Cloverly Formation, as well as, in Tertiary Conglomerates on the west side of the Basin and in gravel beds along river terraces and riverbeds. Agate and Chalcedony nodules are found in the Absaroka Volcanics. Vug: Like geodes, vugs are cavities that contain crystals or cryptocrystalline material. The center of the cavity remains open. Unlike geodes, vugs do not have a competent lining so if they separate from their host rock they do not remain intact. Unlike many geodes, vugs probably will not exist after the host rock weathers and erodes away. Like geodes, thundereggs and nodules, vugs are generally spherical. Miners often call vugs full of large crystals “pockets”. If the minerals have precipitated along cracks, fault breccia, fissures or joints they are generally called fissure or breccia fill. These are long linear crystal veins and are not usually considered vugs. Some rockhounds put a size limit on their usage of vug. Even if it has a competent lining some may call it a vug if it is small in size. As you can see it can be confusing in a conversation between rockhounds. In the Bighorn Basin vugs are found in igneous and sedimentary rocks. In the Bighorn Mountains and Basin vugs containing calcite are common. The Absaroka Mountains have yielded vugs with various zeolite minerals. Thundereggs: This term applies to a specific rock structure that occurs in rhyolite and related volcanic rocks that are formed under certain conditions. The voids where the thundereggs forms are called lithophysae. These are spherical structures (spherulites) formed in rhyolitic igneous rocks that have undergone some gas pocket expansion and growth during a highly explosive volcanic event. This rapid gas expansion creates voids, Lithophysae, in the resultant lava flows. After the lava flow is emplaced, low-temperature hydrothermal solutions and/or groundwater become saturated with silica from the original magma and dissolved from surrounding high silica country rock. The water then transports silica in a dissolved state into the voids. The water solution with its high silica concentration gradually turns into a silica gel which eventually hardens into chalcedony, agate, etc., and sometimes opal, to form thundereggs. Sometimes there is space left in the thunderegg and quartz crystals form in the center. In this case you could call it a geode. Because thundereggs form a certain way, they can be a geode, but all geodes are not thundereggs. Most commonly thundereggs are filled solid with cryptocrystalline quartz and possibly some opal. Rarely, other minerals could be present. Thundereggs can be from a ½ inch to several feet in diameter. We do not have thundereggs in the Bighorn Basin. Thundereggs have been found in Yellowstone National Park but are not collectable. (Rock collecting is not allowed in National Parks) Dig Deeper https://geology.com/articles/geodes/ https://www.mindat.org/glossary/geode Garlick, D. and Jones FT, 1990, Deciphering The Origin of Plume- Textured Geodes: Journal of Geoscience Education, v.38, p. 229 Cocos Geodes, Brazil https://zh.mindat.org/article.php/1098/Thundereggs https://zh.mindat.org/glossary/lithophysae Greg Jones March, 2022 |
JANUARY 2022
Greg Jones 1/22/2022
Dig Deeper
References:
https://serc.carleton.edu/NAGTWorkshops/sedimentary/images/cone.html
https://www.researchgate.net/publication/337874101_A_Possible_Bentonite_Origin_for_Carbonate_Cone-in-Cone_Structures_within_the_Late_Famennian_Bedford_Shale_Rowan_County_Kentucky
https://www.cambridge.org/core/journals/geological-magazine/article/origin-of-spherulitic-and-coneincone-concretions-in-cambroordovician-black-shales-st-lawrence-estuary-quebec-canada/EE767546129236E5406B5F70DFA41AD2
Dig Deeper
References:
https://serc.carleton.edu/NAGTWorkshops/sedimentary/images/cone.html
https://www.researchgate.net/publication/337874101_A_Possible_Bentonite_Origin_for_Carbonate_Cone-in-Cone_Structures_within_the_Late_Famennian_Bedford_Shale_Rowan_County_Kentucky
https://www.cambridge.org/core/journals/geological-magazine/article/origin-of-spherulitic-and-coneincone-concretions-in-cambroordovician-black-shales-st-lawrence-estuary-quebec-canada/EE767546129236E5406B5F70DFA41AD2
FEBRUARY 2022
DENDRITES
Greg Jones 2/22/2022
Dig Deeper
(For more information)
https://www.mindat.org/min-26645.html
https://en.wikipedia.org/wiki/Dendrite_(crystal)
https://www.phase-trans.msm.cam.ac.uk/dendrites.html
https://minds.wisconsin.edu/bitstream/handle/1793/34662/Dendrites.pdf?sequence=1
Dig Deeper
(For more information)
https://www.mindat.org/min-26645.html
https://en.wikipedia.org/wiki/Dendrite_(crystal)
https://www.phase-trans.msm.cam.ac.uk/dendrites.html
https://minds.wisconsin.edu/bitstream/handle/1793/34662/Dendrites.pdf?sequence=1
