Geodes
16 messages in this thread
Hello all!!
The subject of geodes has come up in another thread and I was wondering if
anybody else has taken an interest in them.
I have not found any myself in the field but have studied them.
They are very interesting in that they appear to be formed from the outside
in. They are laid up for the most part in very even layers or rings when you
look at a slice.
A particular layer of similar depth can be traced around the margins of what
can start out as a very irregular interior shape. This layering is remarkably
even and tells me the geode was in constant motion while the layers were
hardening.
Many geodes apparently also came to rest during the last layers to be
deposited. Sometimes the last resting layers are in a couple of different
planes if the geode was disturbed. The level planes indicate that these layers
were formed while immobile as opposed to how the same geodes first layers were
formed.
Some geodes have crystals in the interior where the geode finally came to rest
and cooled slowly enough for the crystallization process to occur.
There is only one explanation of the even coatings of the irregular interior
surfaces and that is that the deposition occurred while under constant random
motion.
I was wondering what other opinions of geode formation there are out there?
Kamron
Kamron:
I'm probably late with this reply but thought it might be helpful. I can't
remember the exact process of geode formation but my old "Sedimentary Rocks"
text says that geodes grow by expansion from cavities in fossils filled with a
common salt solution and isolated by a layer of gelatinous silica. In the
course of time, waters outside the cell freshen, and osmosis builds up internal
pressure, forcing fluids (and precipitation or solidification of the gel)
against the cell walls. Expansion continues until the cell volume reaches
equilibrium with the surrounding fluids. This is followed by shrinkage and
cracking, allowing circulating groundwaters to deposit calcite, quartz, etc. in
horizontal layers.
Frank Ludeman
Never too late to reply
Wow
I guess the author hadn't looked at many geodes.
They definitely build the layers from the outside inward.
It is also easy to tell if they were broken because of the exact record of the
layers. So I can't go with that theory on the geodes in my collection.
I appreciate the input. It is unusual to find writings about geodes. Which
source are you taking that from?
Hi, Kamron–
I got lucky and found the May, 1992 issue of Rock & Gem magazine, which has a
discussion of geode formation (it refers in turn to _Agates_, by Dr. H. G.
MacPherson, and _The Origins of Agates, Thundereggs and Other Nodular
Structures_, by Ben Schaub).
There is some difference of opinion as to whether the blobs of silica gel form
with the igneous rock, or whether they arrive later to fill cavities as I
described. Because it is a gel rather than a liquid, because it generally
cools starting with outside, and because it typically fills the cavity, it
deposits quartz (chalcedony) on all surrounding surfaces with little regard for
gravity. As it continues to cool, layers of quartz are deposited with slightly
different chemistries, thus the banded appearance.
The layers do not form fully solidified, but still somewhat plastic. If the
temperature/chemistry changes to favor rock crystal quartz forming instead of
chalcedony, heat may be released during this crystallization and force the
plastic chalcedony layers to curve toward weak points in the containment,
forming escape tubes (to allow a pressure release or an expansion of the
containment).
Stepping beyond the article…
When crystals form directly on a solid surface, gravity is more or less
irrelevant (gravity dictates the surface of the liquid relative to any
accompanying gas or other liquid phase, thus some rock surfaces can be blocked
off – but this does not appear to apply in most geodes). Gravity governs
crystal formation only when the crystals form in a liquid and then fall thru
that liquid. I figure that sometimes the liquid/solid ratio of the gel may be
such as to leave it more or less a liquid in the late stages, allowing crystals
to form in the liquid and fall to the bottom, forming the flat oriented layers
one sometimes finds inside a geode. This may need a sudden temperature shift.
Eventually dissolved solids may be exhausted, and/or the gel/liquid escapes
through one of these escapement tubes, leaving the cavity often found in the
geode (indeed, many call it a "nodule" rather than a "geode" when there is no
cavity).
–Doug
I appreciate the reference on the article.
I haven't found geodes in the field but they seam to come with just as thin
shell around the quartz deposits.
I must say on reconsideration that there is a lot of evidence in my samples
that could support a gel type of substance as aiding the coating of layers. The
best indicator is where there is an intrusion into the gel it effects the
deposits in that area. I also see some evidence for layers dissolving in places
and being replaced by another material.
This also solves the defect recoating.
Impacts deform the plastic shell cracking it and then new material comes
through and coats through the cracks again.
There are many geodes that have an asymmetric non layered fill also.
Kamron
Kamron
I can't genralize to all geodes, but I have collected a few in the field. What
you find at the couple of sites I've visited is a bunch of globs of silica
ranging from thin shells to solid lumps. I believe that professionals
collectors check the density of the lump and only bother to cut the least dense
— the ones with the largest holes.
Don
Hi Don
What type of sites have you found them in?
Oh Don have you by chance studied gravel under a microscope?
I have recently been reopening a small investigation of gravel.
What I find on gravel that has been removed from a gravel bed, not a river
gravel is an amazingly complex surface that shows no evidence of wear or
friction.
Kamron
>>> What type of sites have you found them in?
I've collected geodes from decomposed igneous rocks in California, and also a
few quartz crystal Keokuk geodes which were formed in cavities in a limestone
in Illinois and Iowa. I'm told that the geodes I collected in California were
formed as Silicate fills of gas cavities in a lava which subsequently
decomposed. No idea how the cavities in the limestone were formed.
My primary interest is fossils, and I see a few nodules and crystal filled
cavities in sedimentary rocks, but the matrix is usually too hard to separate
the potential geode or nodule from the native rock.
>> few quartz crystal Keokuk geodes which were formed in cavities in a
limestone in Illinois and Iowa.<<
Did these have generally uniform bands all around
or were they more horizontal or a combination?
>>formed as Silicate fills of gas cavities in a lava which subsequently
decomposed.>>
Seams odd the lava would decompose leaving the quartz.
I have discovered a lot of quartz gravel has imprints of shells, sometimes the
shells are enclosed within quartz and the body parts replaced by quartz.
I downloaded a Surveyor 1 picture that has what looks like a Kritosarus. Maybe
you could identify. Krit.gif lib 12
Kamron
Hi, Kamron–
Since lava rock is composed of many minerals, the odds are that some of them
will dissolve away faster than the quartz of a geode embedded in the lava will.
Once some of the tiny grains are gone, the structural integrity of the lava
rock is gone, and the "tougher" minerals can be rolled away by wind or flowing
water without any dissolution or other reaction being needed. This breach of
integrity will not happen to the geode; a bit of the outermost quartz may
dissolve, but the inner layers will continue to hold the geode together. The
commonness of quartz in sand suggests that a great many minerals will weather
faster than quartz.
This is my own guesswork; I am not clear on which of the two factors
(structural integrity and rate of chemical weathering) is most important, and a
third may have escaped me.
–Doug
That makes sense.
The quartz does remain, infact you almost wonder where all the other material
went. The world is covered by quartz sand and quartz gravels. All over the
surface. This is what makes the theory of gravel and sands being formed by the
earth belching them out of large fractures that opened and then slammed shut so
attractive to me.
The overall theory can be attacked easily in small areas but it is the total
coverage of all these bits and pieces of not quite explained observations that
brings it home to me.
Kamron
Don
I am trying to gather more data on Geodes
The gelatine matrix makes sense and I have worked with certain silicas that in
the presence of high PH could gel up.
What kind of site was it…
Gravel beds?
Faults or canyons nearby?
Petroleum production nearby?
Thanks for the Data
Kamron
Kamron, I grew up where geodes were extremely common in the limestone bluffs
along the Mississippi river in northern Missouri. Anywhere a small stream
flowing down to the Mississippi cut through the limestone you could generally
expect to find geodes and fossils as the limestone was eroded away. Of course
you have to be the first one to go looking in a given stream bed in order to
pick up the treasures that are right at the surface. -db-….
Richard
Oh I bet that was fun!!!
I have got some petrified wood areas just like that.
Get out there right after a downpour and hit it big time!
Is there gravel and sand also?
Is the limestone bed in thin layers?
Looking at the Geode situation it was definintly a gel that firmed up on at
least the outside. Many were then broken or squashed then sometimes at least a
leak in of materials in places. The change fron the even perimeter layers to a
gravity set of layers is puzzling when the material in the hardened state is
basically the same.
Sometimes they have formed around something like a pearl.
The gel apparently could be dissolved in certain areas near penetrations of
some type and replaced by another cycle of deposits.
I still think that they probably were belched out of the earth like gravel and
sand, in a silicates with ?limestone dust?. don't know.
Kamron
Kamron:
I referenced _Sedimentary Rocks_ by F. J. Pettijohn. Doug's explanation is
much more clear and I like it better. I found my R&M that had the article he
mentioned.
Regards,
Frank L.
Frank
Yes
I see that as a working model for geodes.
Thanks
Kamron