I am running TGA/DTG analysis on a sample of wastes to quantify the amount of calcite in it. In the literature I see that calcite decomposition occurs at T greater than 500 C, but occurs as a single event as evident from a constant slope of the TGA curve (Weight vs T) between 500 and 800 C and a single peak in the DTG curve. On one of my samples I witnessed that there are two events as shown from two peaks at that particular range and from the TGA curve I can clearly see that the slope of the curve changes in that particular range. Does this necessarily mean that there are two different minerals or could the same mineral display more than a single weight loss event?
Thanks
Faisal
A single mineral can show two (or more) peaks in the DTG curve, but calcite shouldn't do it. However, aragonite - a less common polymorph of calcium carbonate - does decompose in two steps, first converting to calcite around 400C with partial decarbonation and then undergoing the usual calcite decarbonation around 700-800C. Dolomite - ideal formula MgCa(CO3)2 - also shows two more narrowly separated peaks in DTG. These are two likely contaminants of calcite, but there are others. You may need to do some elemental analysis - e.g. ICP - and powder XRD to figure out what you have.
Thank you Emanuel Cooper .
I do not expect aragonite nor dolomite to be present since I performed XRD analysis and did not observe any evidence for there presence. A while ago I performed the analysis on the sample and it yielded the ideal result for calcite decomposition, that is, a single peak in the DTG curve. However, it was pointed for me that I need to further reduce the particle size of the sample and redo the analysis and when I did so, I got the two peaks in the DTG curve as described above. Not sure why.
It seems to me that the key is in the details of the size reduction process . I assume you dry-grind the calcite; if so, maybe you generate an amorphous portion of the sample, which should decompose at a lower temperature than the larger grains. I don't know why the discontinuity between the peaks though; usually grinding will not cause a discontinuity in particle size, but it can't be ruled out. You may be able to check that with a particle size analyzer.
Typically, calcite decomposes thermally in a single-step reaction. However, bear in mind that the strongly weathered calcite exhibits a double-stage reaction...
Many thanks Wojciech Bartz THe sample that I studied is a piece of slag that has been weathered for a long period (+20 years). I was wondering if you have a reference that I can read to help me in discussing such findings.
Many thanks.
Faisal Mohammed Try to find out the book written by M. Foldvari (If you need more, please contact me directly via email). There you find information on the double-step dissociation of weathered calcite.
Földvári, M. Handbook of the thermogravimetric system of minerals and its use in geological practice. In Occasional Papers of the Geological Institute of Hungary; Geological Institute of Hungary: Budapest, Hungary, 2011; Volume 213, pp. 1–180
On TGA analysis of calcium carbonate minerals (e.g. calcite) containing iron: https://www.researchgate.net/post/How_would_the_addition_of_metallic_iron_to_a_sample_of_CaCO3_effect_the_TGA_and_DSC_curves_of_CaCO3
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