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LocalitiesJacob's Geode Mine, Hamilton, Hancock Co., Illinois, USA

22nd Nov 2019 20:38 UTCKevin Conroy Expert


I was recently shown the above paper on the minerals found in one geode found at this locality.   This is shared with permission from Dr. Finkelman.

The following slides were all authored by Nova Mahaffey:

22nd Nov 2019 21:17 UTCKevin Conroy Expert


22nd Nov 2019 21:18 UTCKevin Conroy Expert


22nd Nov 2019 21:18 UTCKevin Conroy Expert


22nd Nov 2019 22:59 UTCDavid Von Bargen Manager

Kevin Conroy Expert  ✉️

e following slides were all authored by Nova Mahaffey:
 Where was this abstract presented?

23rd Nov 2019 01:09 UTCKeith Compton Manager

Very interesting
It's a shame the slides are all blurred. Any change of getting the real slides/article for Mindat?
Would make a great addition to Hamilton, Illinois.

Thanks for sharing

23rd Nov 2019 02:19 UTCKevin Conroy Expert


David, it looks like this was presented at the 2019 AAPG SW Convention.   In an email Dr. Finkelman also told me: "We are working on another abstract for a regional GSA meeting and then we will turn our attention to writing a comprehensive paper on the geode."

Keith, I have the slides in Powerpoint format.   I converted them to jpegs and stitched them together, so I wouldn't have to reply to myself X times to show them all.   If you click on the images above you can see them clearly.   I've been given permission by Dr. Finkelman to use the information and presentation as I wish, as long as I give credit of course.   Please let me know how you would like them to be added to Mindat.   To me it would make sense to add the minerals and photos to Jacob's Geode Mine, but I'll put them elsewhere if need be.

23rd Nov 2019 04:18 UTCKeith Compton Manager

I think it would be best made into an article - linked to the locality - thus preserving the format of the orginal

23rd Nov 2019 04:46 UTCD Mike Reinke

Since no two snowflakes are the same, can two rocks be the same?!
Thanks for the information.

23rd Nov 2019 11:44 UTCLarry Maltby Expert

The way that you “stitched” these pages together is very clever. When I click on each strip and then enlarge the picture, the data is razor sharp on my monitor. When I got into this hobby in the 50’s one of the first specimens that I purchased was a Keokuk geode.

23rd Nov 2019 14:29 UTCBob Harman

HI KEVIN, LARRY et al,       Interesting article and nice write up, Kevin!     

The chambers of commerce in the tristate area of Keokuk Iowa got it right when they recognized their geode heritage. They marketed this to the whole Midwest community and made collecting geodes a real tourist attraction. There is a yearly festival with big mineral show and several family friendly fee for collecting sites, including the Jacob's Geode Mine.

For many years geode collecting in the Midwest had been a common family past time. Millions had been collected by youngsters and adults alike.  Of real importance in geode collecting for many youngsters, was the fact that it served as the entrance way to serious mineral collecting and a gateway into the academic field of the geological sciences.

One interesting trivial note is that sooooo many geodes were collected only to be discarded over the years, that geodes are subsequently verrrrry commonly found where they did not originate.

Pictured  are several hi quality examples  from my Keokuk geode sub-collection.

The very large example above is from the original Sheffler's Geode Mine, Clark County Missouri, 1970s.   Manganese rich calcite on light smoky quartz.


23rd Nov 2019 14:33 UTCBob Harman

A 6.5 cm example with a 3 cm double terminated pink "pagoda" calcite on irridescent pseudo-cubic brown calcites. Keokuk, Iowa

23rd Nov 2019 14:38 UTCKevin Conroy Expert

Hi Bob,

Thanks, but all I did was pass the information along.   All of the credit goes to Bob Finkelman and Nova Mahaffey.

23rd Nov 2019 14:38 UTCBob Harman

A small 3 cm geode with the unusual combination of a 1+ cm aragonite spray adjacent to a 1 cm calcite crystal, also with ankerite,  all atop quartz; the very collectible blue quartz variety.  

23rd Nov 2019 15:06 UTCHarold (Hal) Prior Expert

Living on the northern edge of the Keokuk geode zone in the 50's we used geodes for a variety of purposes such as doorstops on farm buildings.  There were several streams that had "geode bars' in the stream bends, instead of sand bars.  In the late 50's my brother and I stopped at a small local stream bridge near New London and filled the trunk of our car in 15 minutes. And left several hundred that could be seen from the roadway. All is closed now to collecting as private property.

23rd Nov 2019 18:26 UTCEd Clopton Expert

I also grew up in Iowa, but farther from the geode territory than Hal.  I've always been aware of them and find them intriguing, but I haven't hunted them in the field.  The U of Iowa Department of Geology has (or at least always had--haven't checked in a while) a fine collection of Iowa geodes displayed in a corridor showcase in Trowbridge Hall.

A small , unremarkable-looking geode fragment in my collection from Schefflers's mine, Alexandria, Missouri, contains at least six macroscopically identifiable species:  quartz, chalcedony, sphalerite, smithsonite, kaolinite, and dolomite.  Who knows what else might turn up under the kind of scrutiny afforded the geode in the abstract?  And the authors chose that one "at random"; imagine what they would have found had they sought out a particularly complex one.

23rd Nov 2019 23:09 UTCFrank K. Mazdab Manager

I'm sorry to be a wet blanket here, but once this gets written up as a paper (if it gets written up), it's not likely to pass peer-review. Their EDS evaluation, is, well, creative.

One of the early slides notes the samples were disaggregated and washed, but they don't describe how this process was done, and in this kind of study that would seem critical (water?, alcohol?, acetone?).  So I wonder how they preserved the halite and especially the sylvite they note in a later slide?  In one slide, the rhodochrosite they point out looks like a surface smudge of dirt (or a hard water deposit left over from the washing with tap water?)... I can believe some Mn-oxide perhaps, but one would think that growing unimpeded in an open space, a carbonate in particular could form at least one recognizable crystal face.  Some of the minerals they mention that would really benefit from an accompanying EDS spectrum (hollandite, goethite [misspelled in most instances... grrr], their REE phase) don't aren't accompanied by one, and some of the spectra they do provide (like for their "Fe-carbonate with Cr, Mo and Ni") are perplexing (that sample, for example, really looks more like an errant chromite grain; their tiny "Mo" bump at ~2.3 keV is almost certainly a bit of S, of unknown origin).  I'm also wary about IDs like "kaolinite", when minerals like nacrite, dickite and halloysite share EDS-similar formulas (indeed, it may even be tough to rule out pyrophyllite, allophane, and maybe 20 other broadly similar compositions if one trying to ID a fine-grained hydrous clay and relying on only Al and Si peaks from an EDS spectrum to do so).

Of course, we can certainly highlight and discuss work like this here, but when it comes to, for example, updating the database with their list of minerals, we need to be a bit more cautious.  And to be clear, because I first thought this was an undergraduate "senior thesis" type of project, where there's an expectation that of course it'll be rough-around-the-edges and that that "roughness" is part of the learning experience for the student, I wasn't originally even going to comment on this.  But I took the time to look up the authors, and the lead author is a staff researcher (but to be fair, a recent graduate of their program), and the second author is actually a career scientist with a specialty in "micro-mineralogy"; those considerations make the problematic issues with this presentation rather more weighty.

23rd Nov 2019 23:53 UTCKeith Compton Manager


To be fair - it's only proposed as a mindat article and they don't need peer review.

I think that an article with this content could be very informative to the average collector. It may  lead to collectors realising that they have additional minerals in their geodes. Maybe even new discoveries or it may even encourage someone to come along and disprove them. It all adds to mindat in a positive way.

There are not many peer reviewed articles in mindat.

As for misspellings, they are on the slides so unfortunately you can't fix that without access to the original.

24th Nov 2019 00:31 UTCFrank K. Mazdab Manager

Hi Keith,

yeah, as I implied in my post, I wasn't trying to be a debbie-downer here... I really hadn't even originally planned to comment on this thread (but alas, I can't pretend I hadn't viewed it... lol).

I had first just glanced at the slides (certainly the concept is quite cool, and also quite plausible) and thought, eh, ok, another kinda iffy EDS study, a bit surprised that the authors missed seeing the various mineral misspellings, but then, it was probably a student's first shot at the big leagues (well, since it's SW AAPG, maybe not quite the big leagues (lol), but a national meeting nonetheless), maybe hastily put together at the last minute (as many presentations inevitably can be, some of mine included), and ultimately it's a learning experience for next time.

But then returning to this thread, I saw there'd been no critical discussion on the content (after all, the whole presentation is right there), saw only "we should feature this as an article" and "just imagine what other geodes will hold?" (with an undertone, in my mind, of "just imagine what MY geodes from there will hold"... making me wonder if "sylvite" was going to start showing up in our Keokuk locality listings?), and then further discovered the authors weren't amateurs or undergrads, but actual professionals.  That all motivated me to be the grumpy curmudgeon I can sometimes be, and say, "hey, wait a minute, folks... let's not be too hasty".

Kevin noted above the authors' plan to write this up and submit it as a paper.  They should.  But it might benefit them to be aware in advance of the kinds of criticisms they're going to get if they are looking at submitting to a real peer-reviewed journal, so they can address those issues in their discussion.  Of course, if they pick one of those pay-to-play journals, I guess it doesn't matter what they write, but that may scare me even more.

24th Nov 2019 01:27 UTCKevin Conroy Expert

I was/am also surprised at some of the minerals reported from this geode.   However, I'm not sure how seriously the Hamilton area geodes have been studied recently, especially given the advancement of the technological tools that are now available to researchers, so there are surely things there that were never detected before.

Also, and I know that I've mentioned this before in a few posts, I have a pet peeve about the use of "REE minerals".   To be fair, in this case the term REE carbonate was used in the abstract, and in the slides the specific REE were mentioned.   Normally when I see REE mineral(s) no further explanation is given.   This leaves me wondering: all of the REE, one of the REE, some of the REE, which of the REE, we're just guessing about the REE, etc.   I guess I'm in the curmudgeon club too...

24th Nov 2019 01:55 UTCFrank K. Mazdab Manager

Hi Kevin,

For me, their use of the generic "REE mineral" doesn't really bother me, because typically all the REE (except Pm, of course) are present in a mineral, and the Levinson suffix we would see in a species name and derived from a detailed analysis just means the REE to the left and right of the named peak are smoothly declining (usually... there are geochemically interesting exceptions).

In this case, using a more precise "LREE mineral" (since they specifically note the notable "light REE" La and Ce contents of their sample) might be more informative.  And as challenging as it is to measure the REE even by WDS, attempting to do so by EDS is a fool's errand.  Indeed, La and Ce are the only two essentially un-interfered REE (un-interfered by other REE, that is... still challenges with Ba, Ti, V, Cr), so had these authors reported EDS-derived Pr, Nd or other REE, I would have called foul on that too.  So glad to see they didn't try that.

And especially if their REE mineral is one of the numerous fluoro/hydroxy-carbonates, I wouldn't expect them to be very successful in characterizing their mineral with EDS anyway... between all the insensitive (or possibly even unmeasurable?) light elements and the overlapping REE, trying to differentiate species from among the whole bastnasite/parisite/synchynite/etc. families would be a huge nebulous mess.

24th Nov 2019 16:49 UTCKevin Conroy Expert

Thanks to all who have commented on this discussion!   Please note that some of the minerals mentioned have only tentatively been identified at this point.   I'm sure that the comprehensive paper on this geode will provide more information for us to enjoy and learn.   The geodes from the Midwest, USA are known worldwide, and going to collect them "in the field" continues to be possible and fun.   I have put together an article containing the abstract and slides above.   If you want to see it go to: 

25th Nov 2019 01:29 UTCPaul Brandes Manager

Thanks Kevin.

25th Nov 2019 02:54 UTCKevin Conroy Expert

Thanks Paul, but I'm just the messenger.   Nova Mahaffey, Robert Finkelman and the geode deserve all of the credit!

25th Nov 2019 08:58 UTCBob Harman

I've done some more thinking about this research and paper's subject.

From a Keokuk geode collector's point of view, I can see it now; I put my Jacob's Geode Mine geode into its next display case and list the 3 obvious minerals that all the show display case observers can plainly see. Then add 10 - 15 probable "invisible" minerals that this research says might be present in my Jacob's Geode Mine specimen.  How great that would be.  All the display case observers are.....well... not impressed.  From a Keokuk geode collector's standpoint, this stuff is meaningless.

How about from an academic relevance point of view?    This research and paper is from the Univ of Texas, Dallas.     Now, if the research came from a evangelical college geology department, then it might really be important.    
The Jacob's Mine geode, from the Warsaw Formation of Mississippian age limestone, has all these minerals present and our "research" clearly shows that it is 10,000 years old!  Now that would be  important!!!   But From the Univ of Texas, Dallas; important  geology department research???       You decide.     CHEERS......BOB

25th Nov 2019 13:35 UTCKevin Conroy Expert

Bob, the logic in this post seems so unsound that I'm not sure where to begin.   I guess the obvious point is to totally discount the notion that a person's geographical location, or the facility that they're at, somehow has an effect on their intellect or ability to conduct research.   It looks to me like the University of Texas at Dallas has the necessary equipment, so unless there is information to the contrary...

Second, minerals identified with high power microscopic techniques aren't going to be visible to the naked eye.   This doesn't mean that they're not present.   If someone labels a specimen according to what might be present without any proof then, well..., they're probably going to be in error. (Note: I edited some text because I made a comment in humor, and later realized that if taken literally it could be construed as insensitive).

25th Nov 2019 13:54 UTCJolyon Ralph Founder

Bob, what on earth are you trying to say there?

25th Nov 2019 18:13 UTCFrank K. Mazdab Manager

Bob, also not sure what your point is given you cheerfully contributed some geode photos earlier and even then noted , "Interesting article and nice write-up...".

If it was intended as some follow-up to my earlier criticism of the research, let me be clear the latter was not related to where the authors did their work, but only that they didn't provide sufficient detail in their slides to discount the possibility that some of their IDs could perhaps alternatively be explained simply by misinterpretation of their EDS data (in general, not an uncommon occurrence) or possibly even a bit of sample contamination. Whether or not those could be issues at play here is independent of where the work is done, but rather with the level of experience of the researchers with the particular instrumentation/samples.  Hopefully once they write their paper and it's been properly peer-reviewed, some of these questions may be answered.

As for labeling "invisible" minerals in a sample, well, that's always a risk in our hobby, with many collectors scrambling to have their own example of the latest rare or unexpected species. As the potential viewer of such a sample in a show display case, or perhaps a potential purchaser of such a specimen at a dealer, all I can say is caveat emptor! As for here, I would hope that on mindat, if such items filter into our localities and photo descriptions, that our many resident experts will continue to ask for defensible evidence of such IDs, and to challenge any questionable or indefensible labelings.

25th Nov 2019 19:27 UTCKevin Conroy Expert

Frank, I've thought about the "invisible" minerals, and realized that I've actually seen this a few times in display cases!   However, I think in all of these instances there were photos of the micro-views next to the specimen(s) in the case, and I think I also recall little leader lines (thread maybe?) showing where on the specimen the pictured area was.

25th Nov 2019 03:42 UTCKeith Compton Manager


But you brought it to mindat

Thank you

25th Nov 2019 13:12 UTCPaul Brandes Manager

And that's what I meant; creating the article with slides for all to read and discuss.

25th Nov 2019 13:36 UTCKevin Conroy Expert

Thanks guys, I really appreciate it!
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