Geodes are geological secondary structures which occur in certain sedimentary and volcanic
rocks. They are themselves of sedimentary origin formed by chemical precipitation. Geodes
are essentially hollow, loosely spheroid to oblate masses of mineral matter that may have
formed from either of two different processes: by the filling of vesicles in volcanic to
sub-volcanic rocks by minerals deposited from hydrothermal fluids or
by the dissolution of sedimentary nodules or concretions and partial filling by the
same or other minerals precipitated from diagenetic water, groundwater or hydrothermal fluids.
Geodes differ from vugs in that the former were formed as early, rounded, structures
within the surrounding rock and are often removed intact, whereas vugs are irregularly
shaped pockets, voids or cavities within a formation, often along a vein or in breccia.
Geodes also differ from "nodules" in that a nodule is a mass of mineral matter that
has accreted around the nodule nucleus. Both structures had the minerals contained within,
deposited from groundwater or hydrothermal processes. Geodes commonly have a chalcedony
shell lined internally by various minerals, often as crystals, particularly calcite, pyrite,
kaolinite, sphalerite, millerite, barite, celestite, dolomite, limonite, smithsonite,
opal, chalcedony and macrocrystalline quartz, which is by far the most common and abundant
mineral found in geodes. Geodes are found mostly in basaltic lavas and limestones. The
Warsaw Formation in the Keokuk region near the area where Missouri, Iowa, and Illinois
join contains abundant geodes.
Formation Geodes can form in any cavity, but the term
is usually reserved for more or less rounded formations in igneous and sedimentary rocks.
They can form in gas bubbles in igneous rocks, such as vesicles in basaltic lavas, or as
in the American Midwest, rounded cavities in sedimentary formations. After rock around
the cavity hardens, dissolved silicates and/or carbonates are deposited on the inside surface.
Over time, this slow feed of mineral constituents from groundwater or hydrothermal solutions
allows crystals to form inside the hollow chamber. Bedrock containing geodes eventually
weathers and decomposes, leaving them present at the surface if they are composed of resistant
material such as quartz. When dissected in half, visible bands corresponding
to varied stages of precipitation may at times show patterns that reveal point/s of fluid
entry into the cavity and/or varied colors corresponding to changes in chemistry.
Coloration
Geode banding and coloration is the result of variable impurities. Iron oxides will impart
rust hues to siliceous solutions. Most geodes contain clear quartz crystals, while others
have purple amethyst crystals. Still others can have agate, chalcedony, or jasper banding
or crystals such as calcite, dolomite, celestite, etc. There is no easy way of telling what
the inside of a geode holds until it is cut open or broken apart. However, geodes from
any one locality usually have a more restricted variety of interior mineralization.
Geodes and geode slices are sometimes dyed with artificial colors. Samples of geodes
with unusual colors or highly unlikely formations have usually been synthetically altered.
Prevalence
Geodes are common in some formations in the United States. They also are common in Brazil,
Namibia, and Mexico. In 2000 a team of geologists found a cave
filled with giant gypsum crystals in an abandoned silver mine near Almería, Spain. The cavity,
which measures 1.8 × 1.7 meters and is 8 meters in length, would be the largest geode
ever found. The entrance of the cave has been blocked by five tons of rocks, and is under
police protection. According to geological models, the cave was formed during the Messinian
salinity crisis 6 million years ago, when the Mediterranean sea evaporated and left
thick layers of salt sediments. The cave is currently not accessible to tourists.
The world's largest known geode is Crystal Cave, a celestine geode 35 feet in diameter
at its widest point, located near the village of Put-in-Bay, Ohio, on South Bass Island
in Lake Erie. See also
Septarian nodule Lithophysa
Thunderegg Bristol Diamonds
References
Further reading Rocks and Minerals, written by Fredrick H.
Pough, ISBN 0-395-91096-X Gerard V. Middleton: Encyclopedia of Sediments
and Sedimentary Rocks. Springer 2003, ISBN 978-1-4020-0872-6, p. 221
Walter David Keller: The common rocks and minerals of Missouri. University of Missouri
Press 1961, ISBN 978-0-8262-0585-8, S. 67 Brian J. Witzke: Geodes: A Look at Iowa's
State Rock Iowa Geological Survey Geodes Kentucky Geological Survey
External links Indiana geode specimens, facts and stories
Video of a geode cracking using industrial soil pipe cutter
Geode images, locations and discussion Australian Museum Fact sheet
Utah Geode Beds Illinois State Geological Survey. "Geodes—Small
Treasure Vaults in Illinois".