Dear Carolina Ortiz-Guerrero , we have used polyethylene XRF sample cups with a transparent plastic film as base (Mylar, prolene, etc) and powdered samples or samples which can be fit inside the cup were analyzed (the later were normally archeological artifacts which couldn´t be milled).

When sample can be transformed into powder we used a conventional pestle and mortar (usually made of agate), with hard materials we use a mechanical mill where both the mortar cup and the milling balls are also made of agate or a material harder than the sample.

The XRF sample cups we used were cilinders of 3cm diameter and 2.5cm heigth, and the amount of sample depends on the fluorescence of it, we often filled the cup at 1.5 or 2.0 cm heigth if we had enough sample. If sample is scarce, there are couplers to reduce the diameter of the cup, so that the X-ray beam passes through enough sample.

About the yellow/orange "alteration oxides", probably it would be better to take fresh (undisturbed) samples from the section, at least if these oxides are formed by exposure to environmental conditions. Keep them if they are already in the fresh samples.

If your former samples were rich on garnets with high content in iron, may be this could be the explanation for the excess of iron detected. May be this excess actually exists, but you must adquire data from a representative sample. If you are milling a piece of material with two large garnets and just a bit of the rest of rock, data will show an excess of garnet chemical components.

Hope it helps.

Hi Carolina Ortiz-Guerrero

Firstly you should select a representative sample and don't sort some particles to get your preferred results. Secondly, in your case, I prefer to use a Fused Beads sample instead of a pressed pellet or powder sample

you can use XRF for evaluating the chemical composition of any unknown material especially if you have Fused Beads general oxides XRF curve to overcome unknown sample properties such as :

Size limits, Physical effects, and homogeneity limit. But the sample needs to be representative.

note: you should apply the attached calculations if you used Fused Beads analysis to calculate the factor (f) for converting test results obtained on the fused basis to analytical results on the as-received basis (by multiplying each result by the factor (f)). The attached picture will help you with calculations.

Hi Carolina,

I have not done a lot of XRF work on samples with large porphyroblasts or phenocrysts so take what I say below with a grain of salt (or crushed garnet hornblende gneiss).

It might help to prepare several aliquots, maybe 7 or 8, from what you believe to be representative subsamples. Then, take equal portions of each and add to an additional aliquot which you you will ultimately fuse. If you want to be extra careful, you can then run each of the 7 or 8 aliquots to see what the range is and how it compares to your "mixed" aliquot sample. While time consuming, I think it is likely that doing this will give you a "representative" sample of the chemistry of the rock.

Ryan