Considering ideal XRD output for Dolomite and Calcite, there is an XRD peak at approximately 2.93 degrees 2-theta to the left (low angle) of each primary line (peak) when the primary line is very tall. Approximately 26.5° for Calcite, and 28.0° for Dolomite (CuKa1 beam).
I've done a little digging and have concluded that these peaks don't match any known minerals from RRUFF or Mindat databases. My thought is that these look strongly like escape peaks one might find in ED-XRF, which I've commonly encountered on a handheld model.
Some publications have identified the calcite-minus-2.93 peak as quartz, probably incorrectly. Other publications have claimed this peak in Dolomite and Calcite spectra are either Vaterite or Quartz. However, these peaks are not accompanied by other Quartz or Vaterite peaks, so I think those publications have made a mistake.
How would I go about mathematically finding the most common escape, pile-up, or multiple-refraction positions? I'd like to see if this offset is both common and predictable with a simple formula.
Stats:
Using CuKa1 beam. The data I am looking at was probably run with a collector of Silicon, but I don't know for certain.
Confirmed: the collector is the LYNXEYE 1D strip detector which is a compound Si detector.
Hello,
Sorry, but I think that's a completely wrong consideration. XRD is not XRF. The Lynxeye detector is partially energy discriminating. It is not possible to subtract energy from the diffraction angle. The radiation has a wavelength of 1.541A (CuKa), energy is approx. 8KeV.
The peak around d=3.343A probably corresponds to quartz, if there are other peaks, mainly with a value of 4.256A. The second peak around 3.18A will probably belong to plagioclase. In natural samples, both additives are common.
I commonly get answers like that, however, the collector surface (SDD) must be considered. We know that the surface of the SSD can be excited and that escape peaks can and will still occur in XRD. I've been told that we're measuring an angle, but we're not. We're measuring a photon, converted to electron pulses, and using that to get an angle. To measure a photon, it must be collected and that in turn can offer multiple modes of error production.
What I found in this case was that the background of the spectra had already been modified. The lab used a CaF2 dope as calibration material, and then the average value of the CaF2 dope was removed from the calibrated XRD histogram. Since the CaF2 was not properly fit, it leaves a high noise value just to the right and left of the point of the CaF2 peaks.
So what we're seeing is the remainder of total XRD minus an assumed CaF2 signature. An improperly fit Gaussian, Lorentzian, or Voigt peak removed from another will leave two smaller peaks at approximately 1 sigma from the centroid. That also explains why the rightmost error is negative and the leftmost error is positive. The CaF2 peak being removed also was offset by up to 10 channels to the right.
The peak doesn't represent any known element because the central part of the peak is now missing. I wish I had known that these samples had been doped and corrected. If other labs have used this same doping technique, then those publications making claims about Vaterite or Quartz signatures at this location are also incorrect.
- Badges
- Science topic
- Similar topics
- Linguistics
- Composition Studies
- Publications