Ca and Mg location in crystal structure ?

Hi,

How can I determine the location of cations such as Ca Mg (a carbonate mineral) ? Are there any techniques to define their location (even their surface abundance on crystal structure is sufficient for me ) ?

Pls note that I have this sample in clay material, meanly it is not "pure". And it seems impossible to clean other impurities ...

Any suggestion ?

Thanks,
Mustafa
Student

There is no way that you can determine atomic positions at home. I would guess you are wondering about telling calcite from dolomite and the like? There are rules of thumb for telling these two apart by comparing the rate of fizzing in dilute HCl. (If you use concentrated acid, the fizzing may be too fast in both to tell them apart.) Don't use nitric or sulfuric acids for this. Calcite fizzes very fast while dolomite is noticeably slower. Dolomite crystals also have greater defects and the "saddle" shape has been used as another rule of thumb, but there are dolomites with few defects and do not have curved rhombohedra. If you want to test for iron, you can dissolve some carbonate and put a drop of dissolved Draino into the mix to see if you form orange iron hydroxide. If you don't have experience with chemical testing, you can get confused over the various results. If you were trying to tell if there were compositional zonation in the carbonate, you have to use very dilute acid spread in a thin film and you have to watch under a microscope, but the acid gets neutralized very quickly and you may still get ambiguous results.

Best Wishes, Van King

Hello,

You can take it to an EPMA (electron microprobe) facility. It is possible to get a element mapping (with spots of a few micrometers in diameter). It will show you the Ca-Mg distribution.

Regards,
Sebastian Möller

Hi Mustafa,

To determine the location of Ca and Mg cations within the crystal structure of a mineral, you would need to use the instrumental mineralogical techniques. X-ray diffraction (one of the methods of X-ray crystallography) of your specimen would provide the answer to your question. If your college/university has a mineralogical department, you can try to find out if they have an XRD facility and if they could do this for you. I hope this will help.

Good luck and best regards,
Goran

To determine average site occupancy for elements in a mineral, the "easiest" way is to do a single crystal X-ray refinement. To map the actual positions of atoms on the surface, you could use an atomic force microscope. Not something that is in every lab. If you are just trying to determine if you have calcite, aragonite or dolomite in your samples, simple powder X-ray diffraction might work.

Thanx a lot for the replies ...

Actually I already applied XRD technique and determined the carbonate species and identified mainly as DOLOMITE (Also, the acid treatment suggested by Van King is very useful and proved Dolomite zonation). But this sample is not pure (with clay materials illite and smectite, but mostly dolomite - %60-70 ). I have some questin marks in my mind because there is no way to study on one dolomite crystal (clayey material), but i will search thoroughly applicability of your suggestions.

I am not a geology student but somehow I have to deal with Ca-Mg distribution (whether regular or irregular) on the cation layer of dolomite ...
I hope I can find the facilities that you suggest , nearby :)

appreciate all your replies ...

Thanks & Best Regards,
Mustafa

In this case Rietveld refinement would be best.