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Techniques for CollectorsCalcite-aragonite test

18th Apr 2010 22:55 UTCReiner Mielke Expert

04438680016015927254494.jpg
The easiest and fool proof test to distinguish calcite from aragonite is using Feigl's solution. A sample of aragonite placed in Feigl's solution will turn black in less than 30 minutes ( at room temp.), whereas calcite will take at least 10 times as long to turn black, if at all. To make sure my solution is still good a always add a known sample of calcite and aragonite as standards when testing an unknown. However I have a solution I made up over 15 years ago and it still works ( you need to protect it from light though).

Feigl's solution is a mixture of silver and manganous sulphate ( 1g/12g) dissolved in 100ml of distilled water. There is a procedure of boiling, filtering and neutralization that is recommended for staining of polished sections but these are not necessary for this type of test, simply dissolve and use.

The photo shows a sample of aragonite ( on the left) and calcite ( on the right) after 30minutes in the solution.

20th Apr 2010 01:09 UTCJohn Duck

I use Meigen's Test. Lightly grind the sample to a powder and boil powdered mineral with cobalt nitrate solution (0.7 gram cobalt nitrate hexahydrate in 10 milliliters distilled water) for several minutes. Allow to settle. Decant solution, leave powdered mineral in test tube. Rinse mineral powder three times with distilled water allowing mineral powder to settle each time to completely remove the cobalt nitrate solution. If aragonite, the mineral powder will be colored light pink or purple-ish pink. If calcite, mineral powder will remian white.

20th Apr 2010 06:13 UTCDonald Vaughn

would not grinding the aragonite to a powder convert it to calcite?

20th Apr 2010 15:43 UTCJolyon Ralph Founder

Why would powdering aragonite turn it into calcite? Unless you powdered it down to the molecular level, it's going to still be tiny bits of aragonite, isn't it?


Jolyon

20th Apr 2010 16:08 UTCReiner Mielke Expert

There was some discussion on aragonite stability on an earlier thread http://www.mindat.org/forum.php?read,11,178577,178657#msg-178657, seems somewhat inconclusive at this point. The Feigl method does not require any grinding so it should not be an issue. Also the method should work by simply placing a drop of the solution on a sample and looking for the spot to turn black. Haven't tried this yet, evaporation may be an issue with this. Will test it tonight and report back.

20th Apr 2010 16:29 UTCJolyon Ralph Founder

I'm not sure how inconclusive the "powdered aragonite turns to calcite" theory is.


From what I can see it's pretty conclusively not true.


I have done a quick search, can't find any reputable source claiming that powdered aragonite turns to calcite, and plenty of scientific papers across many disciplines talking about powdered aragonite.


Is there any genuine research published that suggests this transformation happens? If not let's just consign it to myths and legends.


Jolyon

20th Apr 2010 17:03 UTCUwe Kolitsch Manager

I remember a scientific investigation of (local) temperatures that are reached when dry-grinding oxidic compounds: up to 600°C.

Several sulphides (of Cu but also other elements I think) undergo phase transformation during grinding.


Prolonged grinding turns everything amorphous by the way, independent of hardness, toughness etc.

A lot of nano-scale amorphous powders are created this way (check scientific literature).

20th Apr 2010 17:09 UTCRoger Lang Manager

See here

http://www3.interscience.wiley.com/journal/109776413/abstract?CRETRY=1&SRETRY=0


unfortunately for most of you in german, but there is mentioning of mechanically induced CaCO3 phase transition. There is a caveat that there have been inconclusive reports on this, mentioned obviously in the cited literature No 11-16 .. one is American Mineralogist, JAMIESON, GOLDSMITH, Amer. Mineralogist 45, 818 (1960), another one J.H. BURNS, BREDIG, J. chem. Physics 26, 1281 (1956). The other references are german. Maybe someone who has access can check those so there may be more clarity on this,


cheers

Roger

20th Apr 2010 18:32 UTCUwe Kolitsch Manager

The abstracts:


The grinding of small quantities of either calcite, aragonite, or vaterite in a stainless steel ball mill produces phase transformations. In each instance the end product is an equilibrium mixture of calcite and aragonite. Before the phase transformation calcite to aragonite can take place, a critical amount of lattice distortion, about 1.5%, must be developed. The calcite produced from transforming aragonite appears with and maintains this same distorted lattice. Vaterite is very unstable in the mill and transforms rapidly, first to calcite and then to a calcite-aragonite mixture. Each crystalline form of calcium carbonate is deformed plastically, and in a flowing together and apart of the grains a grinding equilibrium is established with an equivalent particle size of about 860 nm. The precipitated calcite suffered a marked reduction in specific surface area of from 20 to 3 m2/g. This type of grinding equilibrium has a markedly different character from the type proposed for laminar solids such as graphite.


A theoretical approach to the calcite-aragonite transformation is presented for the case of a dislocation glide mechanism obeying first-order kinetics. The kinetic parameters are determined from recent experimental data from the literature; the sensitivity of the model to these and other parameters like the grain size and the deviatoric stresses are discussed.

The model results in the definition of what could be the actual transformation domains of the CaCO3 polymorphs in the (P,T) plane. These domains are different from the classical ones, i.e. different from the stability fields defined with respect to Clapeyron's curve; they can evolve in the (P,T) plane when a deviatoric stress is applied to the system. In this case, we can define a domain in which the two senses of the calcite-aragonite transition are activated simultaneously. The application of the model to laboratory experiments needs the integration of the kinetic differential equation (K varying withP andT). Using this method, we demonstrate the role of (P,T,t) path on reaction rate and the role of experimental uncertainties on the reaction parametersK andn.


Finally, we discuss the problem of the synthesis of polymorphs out of their stability field and of their formation by grinding; these problems, difficult to tackle in terms of diffusion, can be approached with the proposed dislocation transformation kinetic model.

20th Apr 2010 19:36 UTCJohn Duck

Well all I can say is that Meigen's Test works and has worked for years. Obviously not all the aragonite is converted to calcite by simple grinding. With all due respect, I think we are talking about technique here and if you grind your samples to nanometer grain sizes and/or high temperatures you should not be surprised if there is a phase change. The question I have is whether this is really a phase change or whether the specimen is a mixture of aragonite and calcite in the first place or has converted to calcite prior to testing. The studies cited are investigating the phase change phenomenon not sample preparation for conducting Meigen's Test. I suggested Meigen's Test to the readers here because cobalt nitrate hexahydrate is relatively cheap and available for purchase from hobby/chemical supply houses such as United Nuclear. Hobbyists can perform this test themselves to help in sample identification. Silver sulphate and manganous sulphate are more expensive and harder to purchase and a source for purchase of these chemicals was not provided.


Finally, I would like to point out that without simple tests such as the one suggested by Reiner or Meigen's Test, a collector's ability to identify aragonite from calcite is limited to crystal form and cleavage which are often not helpful.

20th Apr 2010 21:31 UTCRoger Lang Manager

Uwe,

thx .. i was able to read the abstracts of course but have no login to read the full articles. The first reference i posted has some interesting conclusions which seem to have been confirmed in later work. If Aragonite is grinded under high pressure the high local temperature will lead to partial phase transition to calcite (as it is the "high temperature stable phase" ) but limited to a 7:3 ratio under the described conditions. Opposite way, calcite will turn into aragonite too because of strain ("high pressure stable phase" ). So Jolyon´s post to put this under myths and legends is certainly rebutted. Still remaining the question what actually happens to aragonite when grinded .... surely depends on HOW the mechanical prep is carried out.


In the other post Reiner mentioned above i reported that we had this issue when grinding (visually identified, automatic mills) aragonite and got calcite peaks too. The reference above would explain that. As you wrote above, sulfides show phase transition due to grinding .. i mentioned the wurtzite-sphalerite problem we ran into. Aragonite grinded manually in an agate mortar - no problem. Another thing is the transition of Aragonite to Calcite in natural environments especially if acqueous solutions are involved. There is a lot of scientific literature dealing with this especially in biogenic carbonate environment (shells, clams) which indicates that aragonite finally being replaced by calcite. Same may occur in hydrothermal environments when a later (hydro)thermal pulse affects early aragonite. So this may explain Reiners and Maggies mexican aragonite issue (or not (:P);-) ).


Reiners described method works well.. we used Feigls solution too. The other method with cobalt salt is better on the distinguishing of dolomite and calcite ... we learned this 20 years ago during a field trip exercise AFAIR.. but no guarantee that i remember correctly , hehe.


(EDIT: may have been wrong remembering .. could have been alizarine red ... argh, i forgot too much stuff)


interesting thread!


And John, full ack! ... but Jolyon questioned the fact of phase transition due to mechanical stress/strain


cheers

Roger

21st Apr 2010 01:16 UTCReiner Mielke Expert

Following up on the Feigl's test. As I suspected evaporation is a problem. The droplet evaporated in about 15 minutes but even then it was able to stain the aragonite grey. In the method I use ( a droplet of solution with a small fragment under the scope) I control evaporation by doing it in a MM box with a lid. I suppose that if I had placed the specimen in a plastic box with a lid that had a wet cloth in it for a while beforehand, evaporation would have not been a problem.

23rd Apr 2010 01:51 UTCJohn Duck

Roger here is the test you are thinking of for the benefit of other readers' interest.


Qualitative test to distinguish calcite and aragonite from dolomite:


Etch a clean rock surface with 10% hydrochloric acid. Place a few drops of Alizarin Red-S (0.1 gram dye in 50 milliliters 1.5% hydrochloric acid) on the area. Calcite or aragonite will stain a pink or red within 2-3 minutes. Dolomite will remain unstained.

23rd Apr 2010 03:28 UTCRock Currier Expert

I would think the reaction rate of 10% HCl on calcite and dolomite would be enough to distinguish those two minerals. Dolomite will react much more slowly than the calcite.

23rd Apr 2010 05:51 UTCVolker Betz 🌟 Expert

Hello,


concerning Calcite/Dolomite : Plain vinegar (5 % Acetic acid) does also the job.


A small grain of calcite will develop significant amounts of CO2 while Dolomite is almost not dissolved.


I do the test under a microscope and use a small particle of pure Calcite (Iceland-spar) for reference. A very small grain <0.5 mm is sufficient.


Volker

23rd Apr 2010 06:16 UTCDavid H. Garske

When I was teaching, I had a grad student give a x-ray demonstration, using x-rays to determine the percentage of calcite and aragonite in a sample. He had ground the samples a couple of years earlier. You can imagine his chagrin when all 10 samples were now 100% calcite.

Dave

24th Apr 2010 09:47 UTCDonald Vaughn

well I had read somewhere concerning the grinding of aragonite converting it to calcite but I can't seem to place my fingers on the specific reference. It would appear that I misinterpreted the means and scale of the grinding as it merely stated that grinding of aragonite can convert it to calcite with little elaboration.

24th Apr 2010 15:27 UTCHenry Barwood

Over the years I've used everything from "Wig-L-bug" grinders to large reciprocating grinders to examine samples of minerals, clays and refractory materials, and can state that prolonged grinding will reduce just about anything to an amorphous state. We are not talking about minimal grinding in a mortar and pestle here, but impact grinding that produces very significant pressure and heat. I don't have the reference, but I remember an article where they ground kaolin for specific intervals and then ran XRD. The diffraction patterns gradually diminished until all you could see was an amorphous bulge.

2nd May 2010 00:53 UTCJohn Attard Expert

I here attempt to find out by actual experiment whether aragonite changes to calcite to a measurable degree when it is ground up as is usual for powder XRD purposes. I took a sample of some white ocean coral bought from a souvenir shop about 20 years ago (probable locality Philippines) ground it to powder in the usual way as is done prior to XRD. This was done by hand in a small alumina pestle and mortar in acetone slurry for about half a minute by which time it was a fine white powder. Then I took another sample if the same coral and ground it the same way but for FOUR times as long. I ran XRD on both samples. There was no noticeable difference in the patterns produced and no peaks developed where calcite would have been expected to produce peaks if any had been formed. Let me mention that calcite has a very large peak at position about 29.5 degrees (expressed as two theta for copper x-rays) . This makes the method very sensitive for calcite.


To elaborate a little further for those interested: A few degrees below that, aragonite has two peaks and they are well resolved from the calcite peak. So by scanning from 25 to 31 degrees one can find aragonite and calcite. Slow scanning and standardization against carefully weighed proportions of pure aragonite and pure calcite enables a calibration to be set up to measure the % calcite in aragonite, a project I did a couple of years ago for a customer. One can measure down to at least 0.2% calcite in a calcite / aragonite mixture. I did not do this calibration this time because it was unnecessary for the present purpose. However I picked up on the experience of two years ago and decided to scan slowly the sample that has been ground for 4x as usual from 25 to 31 degrees. The image shows the XRD scan and reveals no peak at 29.5 where calcite would have shown up. This does not contradict what Uwe mentioned. If I had ground it for a hundred times as long or in a ballmill or at high temperature or pressure cooked in water in a hydrothermal bomb in an oven at 180 C the results could have been different. Here I set to find out whether or not we can be confident that regular powdering of aragonite for XRD testing has the danger of producing false results by generation calcite during the process. I conclude that we can be confident that no calcite is produced by the grinding process under the conditions used.


John Attard

San Diego, California.

2nd May 2010 01:37 UTCRock Currier Expert

John,

That is useful practical information. You should write that up as an article and also include some of the information of others and give the back ground as to why you ran the tests you did. You might also repeat the experiment on a known specimen of aragonite from a well know aragonite locality, like the aragonites from Spain or Morocco. I would be glad to donate a few specimens from Morocco if you don't have one handy. I think it would be us full to post a quick note as to the results of your experiment on the Aragonite locality page with a link to your article? As you know by this time I am not bashful about trying to hornswaggle others into doing work for free.

2nd May 2010 02:19 UTCJohn Duck

John,


Thanks for taking the time to investigate and quantify the effects of hand grinding on the aragonite-calcite phase transition. I think you have laid this concern to rest and provided all of us here with some very useful information.

2nd May 2010 04:30 UTCHenry Barwood

Hi John,


Good test! We used to use large puck grinders at NARCO and those things would grind refractories to an ultrafine powder in under a minute. The pucks weighed about 5 pounds and were Tungsten Carbide. We were mostly interested in pressing XRF pellets, so amorphitization was never a problem.

5th May 2010 05:19 UTCJohn Attard Expert

"As you know by this time I am not bashful about trying to hornswaggle others into doing work for free."


Rock by this time you have donated thousands of hours of useful work to mindat so you need not be bashful at all. So the least thing I can do is to test the two aragonites you suggest: one from Morocco and one from Spain. I did so and both are aragonite; no calcite was detected. (detection limit bolow 0.1%). The scans look like the attachment I put recently on this thread.... no peak at 29.5.


John Attard

San Diego, California.

15th Aug 2010 15:13 UTCKeli

Hello everyone,

i am new in this forum but it is very nice and rich in different sources of information. Really Great forum! But i did not understand something and i would like to ask. I am not too familiar with XRPD (X-Ray powder diffraction) and i do not understand why and how it works the recognition between aragonite, calcite and vaterite? I know that every polymorph has a different 2theta where it appears, but why and how exactly this works?

Thanks in advance if somebody decide to answer me?

Regards,

Keli

15th Aug 2010 16:24 UTCPeter Nancarrow 🌟 Expert

Keli,


When you put a specimen of a mineral into an XRD powder analyser, the pattern of peaks on the 'spectrum' (appearing at the characteristic diffraction angles, the 2-theta values you referred to) is a function of the relationship between the wavelength of the X-rays being used and the spacings between the lattice planes in the crystalline material being analysed.


So, the fundamental thing to understand about X-ray diffraction is that it is a method which gives data derived from the physical dimensions of the crystal lattice planes, rather than being directly related to the chemical elements in the material being analysed. The different arrangement of ions in each different crystal structure gives rise to a characteristic pattern of peaks representative of those structures. Even if the same set of elements is present in exactly the same weight proportions in each phase, so that they have empirically the same chemical compostion (i.e. they are polymorphs of a particular chemical compound - e.g. calcite/aragonite, rutile/antase, kyanite/andalusite, etc), each structure will give a different XRD pattern.


It's a complex subject, but if you want to read a bit more explanation of what I have very briefly described above, have a look at this link: The basics of X-ray Diffraction.


Hope this helps; glad to add more if you have any other questions.


Pete N.

22nd Aug 2010 15:52 UTCKeli

Hi Pete N. ,


thanks a lot for the explanations. They were really nice and clear. I am very glad that i know much more now then before. Thanks once again.

Regards,

Keli

22nd Aug 2010 22:15 UTCWilliam G. Lyon

There is another aspect to the calcite grinding problem mentioned above; that is, the partial decomposition of calcite with loss of CO2. This leaves behind CaO. This product is fairly difficult to detect by X-ray methods for low levels of conversion; however, the reaction of this lime with water causes a huge heat of wetting for the ground powder. This extra enthalpy has been noted by calorimetrists studying the surface characteristics of ground calcite, etc. compared to carefully precipitated products.


The amorphization of quartz by grinding has been studied in many published papers, and its progress can be easily followed by X-ray, FTIR, and other means. Many surface studies of the zero point of charge for "quartz" based on ground material are probably for amorphous silica.

4th Apr 2012 09:11 UTCRalf boustani

Is there any simple way to find out if the calcite was transfered into Argonite .

thank you for your help

Ralf

6th Apr 2012 10:34 UTCFrank K. Mazdab 🌟 Manager

The Feigl's solution method also lends itself well to SEM studies of calcite-aragonite intergrowths (perhaps more of interest to paleontologists and marine biologists than mineral collectors), because the precipitated silver on the aragonite surfaces can be readily detected by BSE imaging or EDS analysis.

5th Mar 2013 17:33 UTCFred Taylor

I have heard this said many times. However, this conflicts with my experience of producing 100% aragonite by grinding recently living coral to a fine powder, sieving it, and running xrd. Zero calcite peaks. We have done this with at least two different corals and found zero calcite. My colleague who wanted his own pure aragonite samples as a standard claims to have ground it for hours greatly annoying his lab colleagues and found absolutely zero calcite in the XRD. I can not explain how others have gotten calcite from grinding aragonite, but we certainly never have. We know our xrd shows a clear calcite peak when even one per cent calcite is added to our aragonite standards so it is not a machine sensitivity issue.

5th Mar 2013 18:38 UTCOwen Lewis

What an interesting thread! It's received wisdom that:


- The nacre that gives mother-of-pearl and pearls their lustre is a layer of aragonitic platelets, laid down over a solid calcitic base.

- Gem coral (Corallium Rubrum) is disordered magnesian calcite with an Mg++ content of ~ 10%, with minor calcium sulphate, phosphates, silica and iron oxide also present.


What we call 'coral' is, of course the excreted exoskeletal material produced by colonies of several species and varieties of marine animal. It's also clear that the exoskeletal material varies in composition according to species. E.g. the main part of black coral (Antipathes grandis and Antipathes dichotoma) is not any carbonate but an organic compound, conchiolin (a protein). Probably therefore, before setting down the chemical composition of these excretions, one may need to be sure too of the animal type that excreted it? It seems that there is no one general case.

6th Mar 2013 17:42 UTCHenry Barwood

Fred,


Was the coral wet or dry ground. I have found that the big WC "puck" type grinders will degrade even clay minerals when run dry. I suspect it is the heat build up and not the actual grinding.

13th Mar 2013 19:14 UTCWilliam G. Lyon

The grinding of carbonates like the CaCO3 polymorphs can also cause chemical changes; in particular, CO2 gas can be released, leaving some CaO. I don't have the reference readily at hand, but this effect was strongly detected in studies of variously prepared CaCO3 powders for which enthalpies of immersion in water were determined calorimetrically. Not surprisingly, traces of CaO on surfaces led to large heats of immersion.

14th Mar 2013 20:07 UTCcascaillou

or let's do it the easy way: Meigen's reaction


I have a good book in french language about qualitative chemical analysis of minerals.


Calcite versus aragonite:

in a pyrex test tube, boil the powder of the mineral for 20 minutes in a saturated cobalt nitrate solution, allow to settle, rince the powder with distilled water, and allow to completely dry on filter paper.

->purple powder = aragonite

->blue powder = calcite (more time required for that coloration to appear)


The book also features three other reciepes (using other reagents) which allow to separate:

-calcite vs dolomite

-calcite vs dolomite vs siderite vs ankerite

-calcite vs aragonite vs magnesite vs ankerite vs dolomite vs siderite

23rd Apr 2013 12:26 UTCAnonymous User

From what a price buy you the cobalt nitrate, because here in Europe it is 200 € 10grammes!


jp

23rd Apr 2013 15:44 UTCcascaillou

I just opened one single french chemical supplier catalogue and found cobalt II nitrate hexahydrate powder 96% purity at 250g for 37euros (which will make about 190mL of saturated solution at 20°C)


and I'm pretty sure I can find much cheaper sources using these two search engines:


http://www.buyersguidechem.com/Asimil.php

http://www.chemexper.com/
 
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