Hi

As I've just tried this I thought I'd write it down, in case of use to anyone else

(1) Load program
(2) Data > Load File (For PFE .dat export file - enter column numbers for x and y - in this case x 23 y 33) > Open slot > Click on each file & repeat open slot (4 andradite repeats, and 4 almandine repeats)
(3) Select 4 andradite repeats - go to Data > Merge Points - output to new dataset
(4) Repeat for 4 almandine repeats
(5) Data > Edit transformations - add new function to normalise data e.g. y=y[n]/max(y)*100 and apply to both merged datasets
(6) Create difference spectra - type in command line @+ =@10-@11 where10 and 11 are the spectra numbers

(7) Duplicate Difference Spectra (see sum button)
(8.) Fit peak to the maximum difference and identify centroid giving a flank position - it only fits peak to positive values so shift spectra to positive values using edit transformations - add new function Y=y[n]+25 - using data-range mode deselect parts of spectra not relevant to fitting - select split pearson7 - auto-add - start fitting - either write down centre of fitting peak (the flank position) or Functions>Export peak parameters

(9) Duplicate Difference Spectra (see sum button)
(10) Fit peak to the minimum difference and identify centroid giving a flank position - use Data > Edit Transformations > Invert - to make negative peak a positive peak - select region for fitting - fit and record peak position as before

You could try simply using the stoichiometric composition of 29.95% C and 70.05% Si.  Use this to see what you get for SiO2 and CaCO3 in-house. 

But as you can see (attached) using EDX would give false C results regardless of how many phases are or are not present.  ONLY SDD can give an accurate EDX analysis for C. 

With respect to this business of phases, it is not worth your time.  It would be more cost effective to go to a jeweller and buy moissanite SiC which is definitely the octahedral phase. 

I appreciate the further insight! We have both powder and single crystal XRD so it may be something worth looking into for us. I think we're just going to go shop around another provider and see if someone has a product we can put some trust in to.

SiC is quite hard to work on even by XRD given so many polymorphs. It is supposed to be a line compound and precipitate out the C or Si depending on what side of the phase diagram you are on, but subtle changes in the C/Si ratio push you into other polymorphs and there is some questions about the stoichiometry of those other polymorphs and is there vacancy ordering etc etc.....

I'm interested enough in the problem that I may sink some time into taking a TEM / atom probe lift out and looking into it further (although each of those have issues too). I think our other possibility is to verify it ourselves using APF factors and pure material standards that we have...Honestly all comes out to the same point - I didn't get what I thought we got

As it turns out the weight %'s they reported are using oxford's factory quant standards inside Aztec...I'm at a bit of a loss what to even do with this block of material now

I talked with our single crystal diffraction expert, Lev Zakharov, and asked him if he could determine the SiC stoichiometry if the materials was single crystal and he responded:

To get the full structure based on X-ray diffraction single crystal method. I can do. Stoichiometric will went out automatically based on the single crystal structure. Powder X-ray diffraction can be used but you need to have powder diffractions from the pure Si and C standards.

Then I asked about the accuracy of such methods and he responded:

I have never been evaluated and never seen such evaluations. From a general consideration I think i t seems to be around 3%-5% if quality of a crystal used for data collection is good.  Typically, 3%-5% is a difference between intensities of symmetrical X-ray diffraction reflections which should be the same based on symmetry.

While our Jeol is being repaired I decided to venture in the cameca instrument we also have for the first time. I am experiencing issues with counting on standards. Peak positions are found effectively but retrieving less than 2k cts. While for Si (Spec 4 TAP-Albite) and Na (Spec 2 LTAP-Albite) this is fine, Al is really bad (Spec 4 TAP - Corundum). I though this had to be with column alignment but then why is it good for some elements and bad for others... Any input will be greatly appreciated.