Looks like some aragonite in there - the hexagonal pillars.

Hello Prof. Alan F Rawle, thank you so much for your reply. Do, you think that the morphology of those primatic crystals corresponds of aragonite? could it be any phase of calcite (as XRD suggested)?

Aragonite is a phase of calcite or at least calcium carbonate.... See, for example: http://www.minerals.net/mineral/aragonite.aspx

Look at the similarity of the crystals in the url.

Calcite, aragontie and vaterite (CaCO3 polymorphs) are typically seen when carbonation occurs in lime/cementitious systems. Would it be possible to post an image of XRD scan? The polymorphs of CaCO3 is easily distinguishable in a diffractogram.

Thank you Prof. Alan F Rawle. Very interesting, actually the morphology looks certainly like aragonite, no doubt, thank you for the link. What it really surprises me it is that on XRD, the calcium carbonate has been always showed as "calcite" and it has been never showed any of the other two polymorphs (aragonite or vaterite). But certainly it looks aragonite no doubt

Sreejith Krishnan Thank you for your comment. XRD results of this carbonated nanolime always says that what it is formed after carbonation is calcite, which it looks very crystaline and fully oriented to {104} (attach the image the XRD, it is named as LAQ, compared to other commercial products which are named NAN and CAL).

Prof. Alan F Rawle,

can those crystals be the calcite "pyramidal hexagonal barrel"?:

http://www.minerals.net/mineral/calcite.aspx

Thank you

@ Jorge Only XRD should provide the real evidence and if your software tells you calcite then I agree that your deduction may be correct. Detailed optical work with a refractometer is another route but if you only have a small amount of material then this may be difficult.

@Alan F Rawle, thank you for your answer. Yes, XRD definitely says that it is calcite (no doubt), but I was surprised since what I saw in SEM wasn't the typical rhombohedral shape of calcite I am used to, it was this shape.

Do you think this particular shape might be due to the preferred orientation of the calcite crystals {104} which was shown by the XRD-Rietveld refinements?

There's a really good section on calcite mineralogy in Dana's textbook of Mineralogy. Somewhere I have a wonderful book called Calcites of New York published in 1910. There are some superb drawings and I would recommend a copy. See: https://ia800206.us.archive.org/23/items/calcitesofnewyor00whituoft/calcitesofnewyor00whituoft.pdf

For more calcium carbonate entertainment see (open in IE):

August 28th, 2012 Optical properties of calcium carbonate

https://www.brainshark.com/malvern/vu?pi=819124498&text=M021507&r3f1=

I wonder how did you employ XRD with such a small sample?

Have you done SAED?

Hello Stefan,

I prepared two samples, one for XRD and another one for SEM. Both were kept in the same conditions (60-80%RH) for 7 days.

After 7 days, I analysed both, and this is the result, XRD says it is pure calcite and Rietveld refinements says that the crystals are fully oriented to {104}, and SEM shows this type of crystals that are not the typical rhombohedral that I am used to see. I was wondering if the prefererential orientation might have an influence on that particular crystal shape.

To see the nanoparticles before carboantion, I just placed 0.12ml of the nanolime solution (Ca(OH)2 nanoparticles dispersed in W/A) over a TEM-SAED samples cupper grid sample holder under nitrogen atmosphere and subquentely I analysed it with the TEM to see the size and shape of the nanoparticles before carbonation.

To see the particles after carbonation, I juts placed the 0.12ml of the nanolime in a XRD and SEM sample holders, and analysed them after 7 days by XRD and SEM.

Hope this heps,

maybe you got mixed phase, which depends on temerature,

can you try performin

Selected area diffraction with TEM?

Hi, Stefan,

What you are seeing in the picture is a SEM image, no TEM image. The size of the carbonated crystals is big enough to see the sample through the SEM.

I only explained to you that I did the TEM for the nanolime before carbonation, where the nanoparticles are so small (20nm) so this is very difficult to see them through the SEM.

I think there is no need to do TEM for the carbonated samples as the crystals can be perfectly seen through SEM.

Calcite crystal : Sys. Rhombohedral (Hex.)

Regards

G.A. Khater, thank you for your reply. Do you think the crystal preferential orientation to {104} showed by the XRD has an influence on the formation of this type of crystals?

Dear Jorge Otero ,

Do you knwo what is SAED?

Dear Stefan, sorry I've just seen your reply right now...the SAED is the selected area diffraction aperture which gives a diffraction pattern, we use it to determine the crystallinity of our samples... and sometimes we used it also to calculate the d-spacing and if our sample is poly-crystalline. However, if the XRD pattern using Rietveld refinements clearly corresponds to the calcite diffraction pattern, and not to other pahse (such as aragonite or vaterite), I don't understand the problem. And XRD also gives you the d-spacing and the crystallinity too. Thus, as far as I understand, I think it is not necessary to repeat the sample using TEM-SAED (but maybe I am missing something).

Can you please explain me how the TEM-SAED analysis will contribute to the characterize the morphology of those calcite crystals please?

no, the measurment of d spacing by XRD and by SAED are not the same values! there are many results that deffer from XRD to SAED, or a present in different ration, which might explain unusual shape of your crystal.

Well, a morphology could create doubt about the attribution of different CaCO3 polymorph, but the XRD pattern doesn't. And in this case, after doing Rietveld refinements, it is clear that it is calcite and there are no other phases around. So, I don't see the discussion about the attribution of those crystals as calcite.

I am focusing now on the characterization of the morphology of those calcite crystals, and if the preferential orientation of them to the {104} (which has been shown by the XRD-Rietveld) might have had an influence on developing this shape, or, it doesn't.

I need X-ray diffraction pattern to identify the type of calcite crystal

that is why you should put one of those crystal, with inconsistent morphology under TEM and do the SAED to see if orientation play any influence.

Sr. Moonmen, the XRD pattern is above

A powder XRD will not tell you thing you look for

I need source file obtained by diffractometry not of a figure .

the wavelength used?

ht

Thanks Sr. Moonmen, but I have no question that it is calcite, as I said above, I am just trying to find if the preferential orientation to the side plane {104} of those calcite crystals can have an influence on the morphology of those calcite crystals, thanks