Hollow formation inside a rock
Not to be confused with
Geoid
.
Quartz
-filled geode, shown from inside (top) and outside (bottom)
A
geode
(
; from
Ancient Greek
γε?δη?
(
ge?d?s
)
'earthlike') is a
geological
secondary formation within
sedimentary
and
volcanic rocks
. Geodes are hollow, vaguely spherical rocks, in which masses of
mineral
matter (which may include crystals) are secluded. The
crystals
are formed by the filling of
vesicles
in volcanic and
subvolcanic rocks
by minerals deposited from
hydrothermal fluids
; or by the
dissolution
of syn-genetic concretions and partial filling by the same or other minerals
precipitated
from water,
groundwater
, or hydrothermal fluids.
Formation
[
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]
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
lava
; or, as in the American
Midwest
, in 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.
[1]
When cut in half, visible bands corresponding to varied stages of precipitation may at times show patterns that reveal points of fluid entry into the cavity and/or varied colors corresponding to changes in chemistry.
Coloration
[
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]
Reddish
chalcedony
geodes
Geode banding and coloration is the result of variable impurities.
Iron oxides
will impart rust hues to siliceous solutions, such as the commonly observed iron-stained quartz. 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 a particular area are usually similar in appearance.
Geodes and geode slices are sometimes dyed with artificial colors.
[2]
Samples of geodes with unusual colors or highly unlikely formations have usually been synthetically altered.
Occurrence
[
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]
Geodes are found where the geology is suitable with many of the commercially available ones coming from
Brazil
,
Uruguay
,
Namibia
, and
Mexico
.
[1]
Large, amethyst-lined geodes are a feature of the basalts of the
Parana and Etendeka traps
found in Brazil, Uruguay and Namibia. Geodes are common in some formations in the United States (mainly in
Indiana
,
Iowa
,
Missouri
, western
Illinois
,
Kentucky
, and
Utah
). Geodes are also abundant in the
Mendip Hills
in
Somerset
,
England
, where they are known locally as "potato stones".
[3]
The term geode generally describes hollow formations. If the rock is completely solid inside, this would be classified as a
nodule
or
thunderegg
.
[4]
Crystal caves
[
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]
Very large
amethyst
and regular quartz geodes
'Crystal cave' is both an informal term for any large crystal-lined geode and also used for specific
geoheritage
locations such as the
Crystal Cave (Ohio)
, discovered in 1887 at the Heineman Winery on Put-In-Bay, Ohio, the
Cave of the Crystals
(Mexico), and the
Pulpi Geode
, discovered in 1999 in Spain. In 1999, a mineralogist group discovered a cave filled with giant
selenite
(gypsum) crystals in an abandoned silver mine, Mina Rica, near Pulpi,
Province of Almeria
,
Spain
. The cavity, which measured 8.0 by 1.8 by 1.7 metres (26.2 ft × 5.9 ft × 5.6 ft), was, at the time, the largest crystal cave ever found. Following its discovery, the entrance to the cave was blocked by five tons of rock, with an additional police presence to prevent looters. In the summer of 2019 the cave, a significant
geotourism
resource and now named the 'Geoda de Pulpi',
Pulpi Geode
, was opened as a tourist attraction, allowing small groups (max. 12 people) to visit the caves with a tour guide.
[5]
See also
[
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]
References
[
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]
Further reading
[
edit
]
- Pough, Fredrick H.
Rocks and Minerals
,
ISBN
0-395-91096-X
- Middleton, Gerard V. (2003).
Encyclopedia of Sediments and Sedimentary Rocks
. Springer,
ISBN
978-1-4020-0872-6
, p. 221 (
restricted online copy
, p. 221, at
Google Books
)
- Keller, Walter David (1961).
The Common Rocks and Minerals of Missouri
. University of Missouri Press,
ISBN
978-0-8262-0585-8
, S. 67 (
restricted online copy
, p. 67, at
Google Books
)
- Witzke, Brian J.
Geodes: A Look at Iowa's State Rock
. Iowa Geological Survey
- Geodes
Kentucky Geological Survey (University of Kentucky)
External links
[
edit
]
Wikimedia Commons has media related to
Geodes
.