I have been having issues with missing peaks in XRD spectrum of Antigorite (and other serpentines for that matter). The best is to illustrate with an example of Antigorite taken from the RRUFF database and one from COD, see the attached picture. You can see that many peaks are missing in the RRUFF spectrum (top). When I measure my sample, I get a very similar spectrum to the one in the RRUFF database rather the one in COD. Especially the intensity of the peak around 36 deg 2 theta is unusual. I tried the other COD samples with similar result (the one in the plot is sample # 9003103.
Can this be related that RRUFF and mine are powder data while the samples in the COD seem to be single crystal diffraction? Or what would be the reason?
Also, similar issues are with samples from the PDF database and all those samples are marked as low quality.
Hello,
you have encountered the classic problems of X-ray diffraction of phyllosilicates, and this is very striking for the minerals of the serpentine group. These are preferred orientation, polytypism and structural modulation.
The upper diffraction pattern shown in the figure cannot be taken seriously, as it only shows basal diffraction (00l). From such a record, it is not possible to reliably distinguish between kaolinite and serpentinite minerals, let alone identify individual members of serpentinite minerals. The reason will be a strong preferential orientation, probably in combination with some structural disorder in other, non-basal directions. The bottom picture looks solid, one could consider the identification, polytypic modification and the nature of the modulation, ie the period of corrugation of the layers in antigorite, but it is probably not possible to determine the powder diffraction.
The issue of structural modulations in phyllosilicates is interesting, but it is almost a separate question. The point is that the structural layers do not fit together completely and are then deformed. In antigorite it is possible to observe ripples, in chrysotiles (similarly to halloysite) there is coiling of layers, etc.
This is usually bad with single crystal diffraction in serpentine minerals. More important for structure descriptions is SEAD/HRTEM.
Dear Leos Pohl
I totally agree with you in that the XRD spectrum of original Antigorite rock is differed from its powder form since the milling process leads to rapid amorphisation and dehydroxylation and results a new structure in short milling time . In [1] they reported that 600 sec of milling in air is enough to proceed such chemical and structural changes. Hope you concentrate on the fact that the dehydroxylation of antigorite powder is closely related to amorphisation, irrespective of the type of mill that is used, which discussed in [1] and related references.
best regards
[1]Article Alkali-Activated Binder Based on Milled Antigorite
Dear Dr Leos Pohl . I agree with
Dr Dalibor Matýsek . He is expertise in XRD.
Dalibor Matýsek Is it worth using XRD data to describe dehydroxylation of Serpentintes or, due to the lack of peaks in the powdered data, it is not a good idea to use XRD with Serpentines at all? (I was hoping to use XRD to confirm the loss of crystal structure and in some cases show that at a certain temperature recrystalization occurs.) Unfortunately, I have no access to SEAD/HRTEM.
Leos Pohl. Hi. I'm sorry, but you're mixing a few issues together. On the one hand, it is a matter of identifying polytypes and modulated structures. This is certainly not realistic with XRD and it will not be easy at all with SEAD. After all, we do not even know how homogeneous real serpentinities are in terms of polytypes and modulations. The second problem will be dehydroxylation, ie how the polytypes are destroyed by heating.
I also don't usually have SEAD or single crystal diffraction available.
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