Yes you are right! With a series of test mixtures as reference, the SCiO should be able to estimate the percentages in a mix.
(Sorry for me misreading your text, and muddying the water with solutions).
Ahhh there is of course room for error with powders of different grain sizes / densities – they will segregate out for fun! The fine stuff ends up on the bottom.
I realise this has drifted a long way from Nikos’ original post theme which was asking about validating sample contributions. This is a critically important subject (Rubbish in guarantees Rubbish out). Consumer Physics did mention this a while ago, and I will raise the subject at the Paris Workshop.