Cochise College
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Roger Weller, geology
instructor
wellerr@cochise.edu
Geodes
Shawn Engle
Physical Geology
Spring 2005
Geodes
Geodes are somewhat of a mystery. Not only is it impossible for one to know exactly
what’s on the inside but also the formation of geodes is still scientifically
unconfirmed. There are many offered
explanations that most accept as possibilities and are tailored to the type of
geode found and its location. Due to
the estimated amount of time it takes for geodes to form, the unpredictability
of numerous geological variables and physical forces that nature has at its
disposal, simulating the same conditions in order to produce a geode is
seemingly impossible to accomplish in one lifetime. However, each geode offers a unique glimpse
of the spontaneous beauty nature can generate with the earth’s minerals.
In order to
attempt to formulate a hypothesis of how geodes are formed, an assessment of
their basic physical properties would be a good start. Geodes are spherical hollow rocks containing
crystalline structures that vary in chemical compound, size, color, density and
internal shape.
.
Picture is property of Shawn Engle for inquires e-mail to
LEGION4WEAREMANY@trustkill.net
But as in nature there are differences and exceptions even
between similarities. The color of a
geode is determined by the impurities the crystals contain, and the geode sizes
range from an inch to several feet. As
majorities of geodes live up to there linguistic Latin roots as being “earth
like” or spherical the geodes found in
Photo property of ZEE’S Minerals used with permission from
Colin Cople. www.zeesminerals.com
Quartz is most common mineral
found in geodes but numerous other types of minerals can fill these
cavities. Some of the most sought after
geodes boast hexagon shaped crystals that line the interior walls of the
hollowed center, ranging from a centimeter in length to several feet depending
on the internal shell size and the amount of time it remained undisturbed. If these types of geodes are allowed to
continue their growth the hollow center will eventually be entirely filled with
quartz or agate. Other geodes get
completely filled with fine-grained minerals that don’t form a hexagon
shape. These form a single solid or
banded
crystalline structure.
Picture is property of Shawn Engle for inquires e-mail to
LEGION4WEAREMANY@trustkill.net
Geodes that contain crystalline
structures have a round hard covering consisting of extremely fine-grained
quartz or silica called chalcedony. With
out this layer the vulnerable crystallized minerals wouldn’t be able to
withstand earth’s weathering process.
Yet in some locations there are other minerals that rank high enough on
“MOH’s scale of hardness” to resist aspects of weathering and suffice as a
protective shell allowing crystals to grow inside. In some cases the original sediment or
igneous material that hasn’t weathered away adheres to this layer of hardened
silica. This is due to cementation
and/or compaction. The pre-existing sediment/country
rock becomes harder than its surroundings and weathers out with the geode then
eventually weathers’ away entirely.
Because this material is clinging to the harder shell it will emulate
the symmetrical shape making it easy to identify.
The proposed process of how
geodes are created is divided into two categories depending on whether they lie
in sediment or igneous rock beds. Geodes
that are found in sediment such as dolomite, calcite and limestone are assumed
to form from either hollow’s made by roots, decomposed animals, abandoned
animal burros and or concretions.
Another suggestion involves cavities that are washed out by ground water
later dry out and harden when the water table drops. These hollows remain full with mineral rich
water or get filled again if the water table rises. This water then with the constant drying out
then re-depositing of mineral water forms the crystalline structures over a
very long period of time. Another way
these crystals form is through a leaching process. As the mineral rich ground water permeates
the hollow, it begins to form the chalcedony shell through silica
deposits. After continual permeation the
remaining deposits eventually form hexagons shaped crystals and entirely fill
the hollow if allowed to grow. On
occasion some geodes that are completely filled have an inner layer of agate
surrounded by the hexagon crystals. This
is thought caused by a silica gel that got through the chalcedony and later
dried.
If the geodes are found in
igneous rock beds the hollows are usually explained as being expanded gas
bubbles, volcanic bombs or igneous mud that contained steam and shrank as it
dried. Lithophysae or thundereggs as they are commonly called are a type of
geode found in igneous rock and have kind of star shaped mineral fillings when
cut. .
Pictures are property of Shawn Engle for inquires e-mail to
LEGION4WEAREMANY@trustkill.net
One theory of how the crystals are formed is silica rich
water interacts with cooling lava creates hexagon crystals or solid chalcedony or agate.
Pictures are property of Shawn Engle for inquires e-mail to
LEGION4WEAREMANY@trustkill.net
Pictures are property of Shawn Engle for inquires e-mail to
LEGION4WEAREMANY@trustkill.net
Other theories most agree on are consistent with the
deposition of silica gel and fluctuating water table. Permeation is also an accepted explanation of
crystal formations.
Geodes are found in specific
areas throughout the continents of the Western Hemisphere and parts of
In
Pictures are property of Shawn Engle for inquires e-mail to
LEGION4WEAREMANY@trustkill.net
Yet the color of quartz varies from crystal clear, to
amethyst (purple quartz). In
Photo property of ZEE’S Minerals used with permission from
Colin Cople. www.zeesminerals.com
A particularly famous location celebrated for its abundance
and diverse geodes is
If one goes
in search of geodes and finds a few the next step is cutting them open. Some use chisels and a hammer while others
use a wet saw. From personal experience
a wet saw is preferable and the type of cutting depends on the type of geode in
possession. The best way to decide on
the angle to cut is cross your fingers and hope it yields the best result. If the types of geode are yielding hollow
cavities lined with crystals I would recommend tumbling the geode around the
blade until a holes are made around most of the chalcedony shell. Then get a screwdriver or chisel and pop it
open, if it doesn’t open then continue to tumble and cut at the shell
eventually it’ll pop open. However if
the geode is most likely entirely filled, chose an angle and drive it straight
through the saw. After the cut polishing
is an option but not necessary because when moisture is applied it looks
polished.
Despite the fact that most
people agree with the proposed formation processes, there are still specimens
and factors that defy these suggested explanations. For instance the giant geode in
References
http://www.isgs.uiuc.edu/servs/pubs/geobits-pub/gep[bot3/geobit3.html , accessed April 15 2005.
http://spectrum.rtoyst.edu/~barwood/indianageode.htm , accessed April 15 2005.
http://www.igsb.uiowa.edu/browse/geodes/geodes.htm
,
accessed
April 15 2005.
http://www.geodegallery.com/keokuk.html
, accessed April 15 2005.
http://www.amonline.net.au/geoscience/earth/concretions.htm
, accessed April 15 2005.
http://www.zianet.com/geodekid/thndregg.htm
, accessed April 25 2005.
http://www.nps.gov/pevi/HTML/geology.html
, accessed April 25 2005.
http://www.rocksforkids.com/R&M/geodes.html , accessed April 15 2005.