For trace element modelling, we need modal mineral abundance of the starting material to calculate bulk Kd. Supposing we have only whole rock composition of the starting material, can we use nornative mineralogy calculated from bulk rock composition to compute the bulk Kd?
Dear Mrs. Chaudhuri,
mineral assemblages based on CIPW norm are artificially created ones and most reasonably applied to the classification of rocks such as in the magmatic camp. If you need a mineral assemblage coming as close as possible to the true one I recommend following the mineralogical pathway either using the classical way by means of the petrographic / ore microscope coupled with point counting and/or XRD. The latter can be used to achieve semi- through quantitative analyses (e.g. Rietveld Method). It is common sense, to obtain reliable results takes time and needs experience. Too often bulk chemical analyses are used to leverage these processes but the uncertainties and deviations from the real mineral associations may be significant.
I wish you much success
H.G.Dill
I can only echo H. Dill's response....nothing can beat doing some old fashioned mineralogy using a petrographic microscope. In my opinion, the CIPW Norm will not result in a reliable modal mineralogy suitable for trace element geochemical modelling.
regards
Jeremy Preston
Thank you so much Prof. Gill and Dr. Preston for your suggestions. Definitely the good old modal calculation is the best possible way to estimate modal mineralogy. But sometimes, while taking some starting material for modelling whose modal mineralogy is not available, then the problem arises. Say I need to apply batch/fractional melting calculation from a rock with only it's bulk composition available in published literature. In that case is it reasonable to try and estimate it's modal mineralogy from it's bulk composition? Such programs are available though.
Dear Mrs. Chaudhuri,
in your case, IMHO the best would be to use "real" mineral compositions of the main minerals truly present in the rock (averages of electron-microprobe analyses, or, if need be, ideal mineral compositions) and (numerically) recombine them to match your whole-rock chemistry.
For this purpose can serve, for instance, the constrained least-squares method. This "chemical norm" (but NB, it is in wt. %) should be much closer to the real abundances of the minerals than any standard normative recalculations, such as CIPW.
This approach is also a viable alternative to the standard point counting techniques, when your rock is porphyritic, fine-grained, the minerals cannot be readily identified under the petrographic microscope or the sample is strongly heterogeneous, e.g. possessing a strong planar fabric.
You may be interested in our, rather simplistic, study:
https://www.researchgate.net/publication/265389741_Distribution_of_elements_among_minerals_of_a_single_muscovite-_biotite_granite_sample_-_An_optimal_approach_and_general_implications
or a book chapter dealing with the least squares:
https://www.researchgate.net/publication/280625885_Mass_Balance_Modelling_of_Magmatic_Processes_in_GCDkit
I trust that this helps,
Vojtech
Article Distribution of elements among minerals of a single (muscovi...
Chapter Mass Balance Modelling of Magmatic Processes in GCDkit
Book Geochemical Modelling of Igneous Processes – Principles And ...
Thank you so much Prof. Janousek, that was really helpful. It is indeed a problem if the rock is fine grained and deformed (say amphibolite). In that case, I suppose, from your answer that a mass balance calculation combining average EPMA composition of minerals with their density and estimated modal abundance, matching whole rock chemistry might help.
I fully subscribe an idea given by Prof. Harald G. Dill. I can help you out in model analysis. If possible, you can use our point counter or weight for couple months time. I am waiting for the installation of Petrog stepping stage. Then, I can analyse your thin sections for model counts.
Best wishes
I fully endorse the suggestions of Dr. Dill and Dr. Vojtech.
Petro-mineralogy is real tool to start with. Normative calculations are indicative in nature and useful for rock classification purpose only. But for trace element modelling, actual modal mineral composition is needed. Prof. Vojtech is pioneer in these fields which is self evident from the attached publications. These may help you to choose the proper methodology to solve the modelling jinx.
With best wishes.
In my opinion, using the mineralogy a petrographic microscope analysis give a good results.