Dissolving limestone to find micro fossils

[Dan 1000]

G'day everyone!

 

I have become increasingly interested in finding conodont fossils and have found a locality near where I live that is rich in paleozoic vertebrate micro fossils, including conodont teeth. The locality is apart of the Coopers Creek Limestone formation, early Devonian in age and rich in carbonate. I have checked out this locality before and the rock is very, very hard (It has no layering and takes a few hard hits from a hammer to smash the rocks open). My question is what is the best way to dissolve these rocks and once dissolved what should I do next to find the micro fossils? I have read some where that hard rocks are soaked in kerosene for 24 hours to break them down but I don't really think it would be safe to use kerosoene and also expensive to buy enough kerosene to soak the rocks.

 

Here is a link to a paper on the site: http://paleoitalia.org/media/u/archives/28___Basden_1999_BSPI_37_527-541.pdf

 

Thanks,

Dan

[Ludwigia]

If the microfossils are silicified, and only then, you could place the limestone in a 10% solution of hydrochloric acid. Of course you should follow the usual safety precautions here. Protective clothing, gloves, goggles, etc. If you need to mix the solution, then add the acid slowly to the water and not the other way around, otherwise you're in for an explosion. Keep a close eye on the progression of dissolution, rinse the remains afterwards thoroughly in very soapy water and let them sit in fresh soapy water overnight to neutralize the acid. Then rinse them again in fresh water and study them under the microscope.

[TqB]

5 minutes ago, Ludwigia said:

If the microfossils are silicified, and only then, you could place the limestone in a 10% solution of hydrochloric acid. Of course you should follow the usual safety precautions here. Protective clothing, gloves, goggles, etc. If you need to mix the solution, then add the acid slowly to the water and not the other way around, otherwise you're in for an explosion. Keep a close eye on the progression of dissolution, rinse the remains afterwards thoroughly in very soapy water and let them sit in fresh soapy water overnight to neutralize the acid. Then rinse them again in fresh water and study them under the microscope.

 

I think conodonts are OK with acid - never tried it though.

from https://www.ucl.ac.uk/GeolSci/micropal/conodont.html

 

"Preparation Techniques

Since conodonts are resistant to mechanical and chemical attack preparation techniques can utilise acids such as acetic, formic, or monochloric to release the elements from their host rocks, which are commonly carbonates. Conodonts are commonly between 200 microns and 5 millimeters in size and can be sieved from finer materials and further concentrated by heavy liquid or ultrasonic techniques."

[DPS Ammonite]

I have heard, from multiple of references, that you should not use HCl on phosphatic/vertebrate fossils since they might be damaged. Most conodonts are phosphatic and are not fully silicified. Try acetic acid instead.

 

The secret is to only use pieces with lots of visible conodonts since the rock will need to be crushed, dissolved in acid and then wet sieved. 

 

http://oceans1.csusb.edu/cdont_art.htm

[FranzBernhard]

31 minutes ago, DPS Ammonite said:

that you should not use HCl on phosphatic/vertebrate fossils since they might be damaged.

Not only might, they are slowly dissolved by HCl! 

 

Wikipedia conodont page gives 3 citations for conodont preparation, one of them is this
https://link.springer.com/chapter/10.1007%2F978-94-017-0581-3_27

Maybe this is helpful.

Franz Bernhard

 

[Bullsnake]

Another thought, if there is erosional material from the outcrop, you might scoop some of that up and gently wash/sift and dry the matrix to search through. I think at least a 10X magnifier should be used to look.

Good luck!

[Guest11596]

If the lithology aligns at your outcrop, the paper at the link below might prove useful to you. From the link below click on the image thumbnail and choose 'Lower Devonian conodonts from the Tyers area, Gippsland, Victoria' from the 'Table of contents' (excerpt below).

 

http://cedric.slv.vic.gov.au/R/N2CML2R1MP8KN4QIL4JPS4JYGXEACM6LULQFPHXEKYGAVDE4GJ-01515?func=results-jump-full&set_entry=000017&set_number=001229&base=GEN01

 

"Abstract

Thirty-four disjunct conodont species extracted from the Coopers Creek Formation, Tyers district, Gippsland, are described and figured. New taxa proposed are Belodus resimussp. nov., Eognathodus sulcatus gen. et sp. nov., Eognathodus secus gen. et sp. nov., Paltodus valgus sp. nov. The known ranges of species common, to the Tyers fauna and Europe indicate an Upper Gedinnian or Siegenian age for the Coopers Creek Formation."

 

"Method of Study

The conodonts described in this study were extracted from limestone collected from the old Tyers Limestone Quarry, on the E. bank of the lower Tyers R., Gippsland. This is given as fossil locality No. 11 by Philip (1962, Fig. 1). The different lithologies exposed in the quarry were sampled. These ranged from mid-grey bioclastic calcarenites to very dark grey calcilutites. A total sample of some 20 kg was collected and processed.

 

The limestone was digested in a 10 to 15% solution of commercial acetic acid. The insoluble residue was washed and screened under water. Conodonts were contained in the material which passed through a 22 mesh-per-inch screen but which was retained on a 120 mesh-per-inch screen. This residue was dried and placed in tetrabromethane (diluted with alcohol to a specific gravity of 2-75). The portion of the sample which sank in this heavy liquid was washed in alcohol and dried, and the conodonts removed under a binocular microscope with the aid of a fine sable brush. Specimens were then mounted on microslides with gum tragacanth to which chlorocresol had been added to prevent fungal growth.

 

In all, some 600 identifiable conodonts were recovered from the 20 kg of limestone processed. The average abundance of 30 per kg is similar to the abundances given by Collinson (1963) for N. American Devonian and Carboni ferous limestones, and compares very favourably with many other Australian Devonian limestones which have been processed. Certain Ordovician limestones from Central Australia, however, have yielded an estimated 30,000 conodonts per kg. This prolific occurrence is probably the greatest abundance of conodonts yet reported from a limestone. Second to it is the record of Bischoff and Ziegler (1957) who found a concentration of over 23,000 specimens per kg in a limestone in the lower Cheiloceras Stufe of Germany.

 

The illustrations were obtained by using a ‘Leica’ camera adapted to give a negative diameter of X 10 with a 40 mm lens. Text-figures were prepared from specimens with the aid of a binocular microscope with a graticuled eyepiece and also from photographs. All photographed specimens are registered in the Palaeon tological Collection of the University of New England, Armidale, N.S.W."

 

This paper seems to compliment the above nicely (link below)

 

https://www.researchgate.net/publication/232877167_The_Early_Devonian_Coopers_Creek_Limestone_A_deep-water_redeposited_limestone_in_the_Melbourne_Trough_southeastern_Australia

[Mediospirifer]

I've dissolved limestone for conodonts. (North Evans limestone, Devonian of NY)

 

I haven't tried concentrated acids. What I've done is soak the pieces in vinegar overnight, then rinsed them in a pan of water. There tends to be a thin film of muddy residue on the rocks, which I remove in the rinse bath with a soft natural-hair paintbrush. A SMALL paintbrush; no larger than 1/8" diameter: the fossils may get caught up in and lost in anything larger. Once clean, the rocks go back into a pot of fresh vinegar for another round of soaking.

 

The residue in the bottom of the wash tub is where the fossils are. It also has a lot of sand and finer particles of mud. The finer fractions can be separated by mechanical agitation: Stir up the water (gently shaking the tub works fine, or adding more water after the pour-off stage) until it's cloudy. Wait a few seconds (count to ten), then pour off about 3/4 of the water. Refill and repeat until most of the particulates settle out quickly.

 

I've found that coffee filters work well for catching particulates. If I pour limestone residue through a coffee filter without separating off the mud first, I end up with a filter full of mud and a bottle of clear water! 

 

Try this out, and see what the unsorted residue looks like under the scope. My first attempt produced a few visible without sieving.

 

There are a number of posts in the Microfossils forum about techniques. I recommend these in particular for low-budget handling of specimens: LINK, LINK, and LINK.

 

Good luck, and have fun!

 

[Mark Kmiecik]

AND DON'T BREATHE THE FUMES, NO MATTER WHICH ACID YOU USE.

[Mediospirifer]

11 minutes ago, Mark Kmiecik said:

AND DON'T BREATHE THE FUMES, NO MATTER WHICH ACID YOU USE.

 

Good point!

 

I recommend using a plastic tub with a lid, in a reasonably well-ventilated area. Don't bother to seal the lid, the amount of gas produced will pop it off! Just place the lid on top to contain any splashing (and keep dust or bugs out).

 

I like vinegar (5% acetic acid) as a solvent for a number of reasons. It's easy to come by, not too expensive, nontoxic, and weak enough that as long as I'm not washing my hands in it (and remember to wash my hands when I'm done) I'm safe. It's slow going, but with repeated overnight soaks it will do the job.

 

One caveat: Don't leave it soaking for more than a few days. I once found a small tub that had been overlooked when I got distracted by other projects, and it was full of mold!