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Calcite-aragonite test

Posted by Reiner Mielke  
Reiner Mielke April 18, 2010 10:55PM
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.
open | download - Aragonite-calcite test.jpg (68 KB)
John Duck April 20, 2010 01:09AM
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.
Donald Vaughn April 20, 2010 06:13AM
would not grinding the aragonite to a powder convert it to calcite?
Jolyon & Katya Ralph April 20, 2010 03:43PM
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?

Reiner Mielke April 20, 2010 04:08PM
There was some discussion on aragonite stability on an earlier thread,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.
Jolyon & Katya Ralph April 20, 2010 04:29PM
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.

Uwe Kolitsch April 20, 2010 05:03PM
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).
Roger Lang April 20, 2010 05:09PM
See here

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,

Uwe Kolitsch April 20, 2010 06:32PM
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.
John Duck April 20, 2010 07:36PM
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.
Roger Lang April 20, 2010 09:31PM
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


Edited 1 time(s). Last edit at 04/20/2010 10:34PM by Roger Lang.
Reiner Mielke April 21, 2010 01:16AM
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.
John Duck April 23, 2010 01:51AM
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.
Rock Currier April 23, 2010 03:28AM
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.

Rock Currier
Crystals not pistols.
Volker Betz April 23, 2010 05:51AM

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.

David H. Garske April 23, 2010 06:16AM
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.
Donald Vaughn April 24, 2010 09:47AM
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.
Henry Barwood April 24, 2010 03:27PM
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.

Henry Barwood
Troy University
Troy, Alabama USA
John Attard May 02, 2010 12:53AM
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.
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