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Calcite-aragonite test

Posted by Reiner Mielke  
Rock Currier May 02, 2010 12:37AM
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.

Rock Currier
Crystals not pistols.
John Duck May 02, 2010 01:19AM

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.
Henry Barwood May 02, 2010 03:30AM
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.

Henry Barwood
Troy University
Troy, Alabama USA
John Attard May 05, 2010 04:19AM
"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.
Keli August 15, 2010 02:13PM
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?
Peter Nancarrow August 15, 2010 03:24PM

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.
Keli August 22, 2010 02:52PM
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.
William G. Lyon August 22, 2010 09:15PM
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.
Ralf boustani April 04, 2012 08:11AM
Is there any simple way to find out if the calcite was transfered into Argonite .
thank you for your help
Frank K. Mazdab April 06, 2012 09:34AM
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.
Fred Taylor March 05, 2013 05:33PM
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.
Owen Lewis (2) March 05, 2013 06:38PM
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.
Henry Barwood March 06, 2013 05:42PM

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.

Henry Barwood
Troy University
Troy, Alabama USA
William G. Lyon March 13, 2013 07:14PM
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.
cascaillou March 14, 2013 08:07PM
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

Edited 8 time(s). Last edit at 03/14/2013 08:23PM by el cascaillou.
Anonymous User April 23, 2013 11:26AM
From what a price buy you the cobalt nitrate, because here in Europe it is 200 € 10grammes!

cascaillou April 23, 2013 02:44PM
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:

Edited 8 time(s). Last edit at 04/23/2013 03:58PM by el cascaillou.
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