Learning Center What is a mineral?The most common minerals on earth Information for Educators Mindat Articles The Elements The Rock H. Currier Digital Library Geologic Time

Minerals by Properties Minerals by Chemistry Advanced Locality Search Random Mineral Random Locality Search by minID Localities Near Me Search Articles Search Glossary More Search Options

The Mindat Manual Add a New Photo Rate Photos Locality Edit Report Coordinate Completion Report Add Glossary Item

Mining Companies Statistics Users Mineral Museums Clubs & Organizations Mineral Shows & Events The Mindat Directory Device Settings The Mineral Quiz

Photo Search Photo Galleries Search by Color New Photos Today New Photos Yesterday Members' Photo Galleries Past Photo of the Day Gallery Photography

Discussions

💬 Home 🔎 Search 📅 Latest

╳Discussions

💬 Home 🔎 Search 📅 Latest

Groups

Recent Images in Discussions

GeneralChalcedony Pseudo after Aragonite, Arizona

6th Jun 2014 16:00 UTCDennis McCoy

This is a new acquisition for me, a chalcedony Pseudomorph of an aragonite cluster. I have seen photos of smaller pseudo aragonite specimens, but large ones seem to be rarer. Actually, I have never seen an aragonite cluster this large.

The primary mineral appears to be chalcedony, with several other minor minerals present. With magnification, there are small areas that may be the original aragonite.

This is reputedly from Arizona, but has no specific origin.

6th Jun 2014 16:05 UTCDennis McCoy

This pseudo is a chalcedony cast of the original aragonite, and since it is hollow could probably be considered a geode.

Could it also be considered to be a fossil? (A fossilized mineral cluster) :-)

6th Jun 2014 20:39 UTCDavid Von Bargen Manager

When you have a hollow cast, the term is epimorph.

14th Jun 2014 21:57 UTCDennis McCoy

Thanks for the info David.

I have been doing more research on this, and have found almost nothing. I have found numerous copper after aragonite examples, but they are much smaller. The only other psuedo from North America (found online) is a larger plate of chalcedony found in Texas. It does not have the cluster arrangement, and looks more like a psuedo of fluorite or calcite cubes to me.

I also have not found photos of any aragonite clusters with this size of crystals. The only specimens close are some of the single crystals coming out of Spain. Are these crystals found as clusters and broken up, or have they formed individually?

Does anyone have photos of large aragonite clusters?

14th Jun 2014 23:25 UTCBob Harman

DENNIS, As DAVID V B discusses, your aragonite example is a cast, having nothing to do with a geode other than both having hollow centers. Aragonite, I believe, is a crystal polymorph of calcite, both being CaCO3. In Indiana both calcite and aragonite are quite common. I literally have hundreds of examples of all types of aragonite expression in Indiana geodes. I have pictured several differing examples in the "Midwest geode" thread. If you are interested in seeing these various various expressions of aragonite in Indiana geodes, I suggest you scroll thru the pages of that thread. CHEERS…….BOB

15th Jun 2014 00:33 UTCDennis McCoy

Bob, I know geode are your specialty, and would appreciate your explanation of the differences between a geode and a cast.

I was under the impression that a geode was a cast of a void in a substrate, such as rock or clay. Geodes are also sometimes fossils aren't they?

15th Jun 2014 02:05 UTCDana Slaughter 🌟 Expert

Hi Dennis,

I've heard of similar specimens being found "near Lake Pleasant" in Arizona.

15th Jun 2014 02:29 UTCDennis McCoy

Thanks Dana!

15th Jun 2014 03:21 UTCAlfredo Petrov Manager

That is an impressive specimen, Dennis! Similar sized chalcedony pseudomorphs after aragonite come from Argentina - See for example the photo: http://www.mindat.org/photo-432697.html - but these are mostly solid replacement throughout, not epimorphs, so yours is different in that respect.

As for the meaning of "geode", I personally avoid the word, as it has too many different uses. In spanish the word "geoda" is even looser than in english, basically just meaning any size open vug in a rock.

15th Jun 2014 03:32 UTCBob Harman

I think it is not so easy to SATISFACTORILY try to explain the difference you request; just that even with the broadest definition of a "geode", I have never considered it to include a "cast". To me they are just basically different. A mineral such as aragonite covers a preexisting mineral as a thin crust and then the preexisting underlying mineral gradually dissolves away so there is just the remaining thin crust forming the cast.

The broadest definition of a "geode" might include various types of sedimentary and igneous lava tube vesicles, nodules and concretions and agates with a hollow center and then to further the muddy waters of this definition there are many solid geode forms as well as the usual hollow examples. For example the Brazilian "geodes" are really not as they are quartz (variety amethyst) lined vesicles in lava tubes secondarily cut to resemble a geode.

The usual more narrow definition might be a sub spherical rounded mass of either igneous or sedimentary origin containing a solid or hollow interior lined by one of several minerals such as quartz, celestine, or calcite (the crystals of which generally pointing toward the center of the cavity) contained within a surrounding rock matrix of differing composition.

Sedimentary geodes, which I am most familiar with, apparently form in several possible ways. One theory is that decaying organisms in the hardening mud, destined to become the limestone matrix, gradually leave a void and silica in the surrounding solution becomes the quartz lining. If the original organism still is recognizable then we have "geodized fossil" while non recognized examples are just "geodes". The other mode of formation adds that the a void in the hardening "proto limestone", NOT necessarily be a decaying organism. This focus contains anhydrite which is eventually dissolved away and replaced by silica…..the geode quartz lining. Perhaps both these modes of formation occur???

If all this sounds a bit cumbersome and gobbledygook, well it even sounds like that to me as well!!! Like a rose is a rose is a rose; a geode is a geode is a geode! CHEERS…….BOB

15th Jun 2014 05:55 UTCD Mike Reinke

Dennis,

I noticed that Bob several times mentioned 'lining'. So differentiate that a geode has a rind, whereas a vug is just a cavity. Vugs can be circular and full of minerals all the way around, but that is not make a rind. The rind separates it from the parent rock. Make sense? I don't know that that is 'textbook' definition, but it separates them for me.

15th Jun 2014 09:34 UTCErik Vercammen Expert

The area of Big Bend in Texas has similar pseudos of quartz after aragonite.

15th Jun 2014 20:47 UTCDennis McCoy

Thanks all for the input. Your suggestions have helped me to find several similar examples, and more not so similar.

I found an example sold in 2011 from the David Stoudt Collection from Argentina that was about the same size.

The psuedos from the Big Bend, Texas area all seem to have a chalcedony coating which somewhat obscures the shape.

There are smaller specimens from the Lake Pleasant Az region which have the similar sharp crystal shape and coloration. My guess is that my specimen is from this region.

24th Jul 2014 15:49 UTCDennis McCoy

I have been searching for aragonite specimens of the same general size and structure of this psuedo (epimorph). I haven't been successful thus far. All of the radial clusters I've found are much smaller in cluster and crystal size. The specimens from Spain are the size of this psuedo's individual crystals, but seem to form in large plates or un-structured masses. does anyone know of specimens of aragonite with this general size and shape? Is there some special condition needed for them to form?

This photo includes a small iron stained cluster from Morocco and a larger clear example from Spain.

25th Jul 2014 06:57 UTCRock Currier Expert

Dennis,

Did you check out all the images here on Mindat for aragonites? We have more than 2000 of them that you can find here::

http://www.mindat.org/gallery.php?min=307

If you can't find some possibilities here they may not exist. I have seen some quartz or chalcedony pseudomorphs after aragonite that look a bit like the cast from Spain that you pictured in your post.

Mindat.org is an outreach project of the Hudson Institute of Mineralogy, a 501(c)(3) not-for-profit organization.
Copyright © mindat.org and the Hudson Institute of Mineralogy 1993-2024, except where stated. Most political location boundaries are © OpenStreetMap contributors. Mindat.org relies on the contributions of thousands of members and supporters. Founded in 2000 by Jolyon Ralph.
Privacy Policy - Terms & Conditions - Contact Us / DMCA issues - Report a bug/vulnerability Current server date and time: April 24, 2024 02:13:31

Go to top of page