Carbonate deposits resulting from a microbial process in a solution containing required nutrients for bacterial growth, 0.1M CaCl2, 0.78M urea and 50% seawater (composed of 0.055M Mg2+, no Sr) were examined using XRD and FTIR (the precipitates were earlier filtered, rinsed with water, dried at 50C). the results are attached.
Looking at XRD, calcite and aragonite phases can be identified, as expected to occure; BUT the IR spectra represents hydromagensite (http://rruff.info/Hydromagnesite) which is in contrary with the XRD result and not expected (as [Ca2+]>>[Mg2+]). Excluding the peaks at 798cm-1 and 590cm-1, we can conclude a mixture of Mg-incorporated calcite (considering a shift in major peaks of calcite to higher frequency due to smaller atomic radius of magnesium ion) and aragonite; but what can we say about these two peaks, why are they appeared? can they be attributed to the other carbonates precipitated in the solution?
Thanks in advance,
update 1-------05.21.2015 9:00am
the XRD peaks of the sample: 2theta= 23.03, 25.99, 26.26, 27.26, 29.34, 36.1, 39.45, 43.27, 47.47, 48.50, 57.46
the IR peaks of the sample: 1480, 1417, 1117, 884, 852, 798, 745, 713, 700, 666, 590 cm-1
considering the peaks and the following references,
Q1- does the XRD spectrum represent a mixture of aragonite and calcite? (if so, answer Q2; if not, what does it show?)
Q2- is the IR spectrum confirms XRD result? (if yes, what are the other appeared peaks (specially 798cm-1 and 590cm-1)?; if not, which compound does it represent? hydromagnesite? if hydromegnesite, answer Q3)
Q3- if the IR spectrum corresponds to hydromegnesite, why the XRD does not show its characteristic peaks? and can we expect to have hydromagnesite in the solution with [Ca2+]>>[Mg2+] ?
http://rruff.info/Calcite/R050128
http://rruff.info/Aragonite/R040078
http://rruff.info/Dolomite/R050370
http://rruff.info/Magnesite/R040114
http://rruff.info/Hydromagnesite
Dear sir,
Please look at the update in description of my question, which I did after your post.
As you see, the characteristic peaks of hydromagnesite (at 2theta=30.6, 15.16) did not appear in the XRD.
Thank you very much for your answer sir,
I made a mistake, the characteristic peaks of aragonite at 26.26 and 27.26 (3.40Å and 3.27Å) have also appeared but are not as prominent as the peaks of calcite.
Does incorporation of a small amount of magnesium into calcite lattice influence the XRD pattern?
I also attached electron micrograph of the sample here in case if it helps to your interpretation.
Thanks again,
Hello Hamed,
Can you confirm a 103.44% at 851.89 cm-1? (Graph is not so large). Do you have any settling during measurement? I have also observed amazing peaks in such situation.
Give me more information about your measurement process please.
Hello sir,
yes that's right; we did not do any specific correction. We did not consider the transmission percent. we are only looking at the wavenumber at which the peaks occur. We used ATR sampling without any pretreatment.
Hello Hamed,
You didn't really answer my question. Did you observe settling of the sample in the spectro cuvette? So, no transmission greater than 100%?
No pretreatment: do you mean that some living matter (bacteria) could be present in the sample? This can induce extra bands and unexpected interactions between particles. (It seems also that the spectrum is noisy. Do you believe that also?)
Dear sir,
As I mentioned in the question description above, we did the test directly on solid precipitates (without pretreatment with KBr...) which were earlier filtered from solution (using ashless filter paper), thoroughly rinsed with water, and dried at 50C.
I can not understand what do you mean by settling in the cuvette! which cuvette?! there is no liquid sample! could you please explain more?
OK. I am usually working in NIR in an aqueous sample. You work immediately on a solid sample, isn't? So, my question doesn't apply. Sorry. But the question of the sample preparation remains. Can you better eliminate what you don't need to mesure?
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