Dear Researchers,
I am working on a petrographic analysis, and I have a thin-section database, referred to partially metamorphic rocks such as meta-basalt, meta-andesite, meta-gabbro and so on.
In the thin-section results, I have got the percentage of several main minerals such as Quartz, Epidote, Feldspar, Chlorite and some other minerals which are significantly varying among samples from one to another. For example, Quartz is fluctuating between 0 and 59 percent in various samples.
In my research, I need to categorize the mineral percentages in three ranges: Low range, Middle range and High range. For example, when we say that there is a high quantity of Quartz in a metamorphosed rock, what exact percent we are dealing with?
I would appreciate if you could share your ideas about this question.
Best,
Behzad
That is a strange question. As Ioan Pintea suggested, 100% is a high quartz value for a quartzite, but 59% is a high value for your meta-basic rocks, but 59% might be a low value for a quartzite. The categories low, medium and high will be different for each different rock. You need to understand the purpose for making these categories. Is there any purpose? If not, why are you doing it?
I agree with the others. Deciding that a metamorphic rock has a high/medium/low content of a given mineral based on percentages is quite arbitrary. If you want to divide your dataset in three ranges, this might be fine, but I think the values you are going to decide will have a meaning only for the specific dataset you are examining.
I suggest to use the manual for the description issued by the British Geological Survey for metamorphic rocks of almost all facies types and parent materials (Robertson 1999). There percentage values have been given for ortho- and parametamorphic and for regional metamorphic, contac metamorphic and dynamometamorphic rocks. Sometimes, it looks like a bit too academic especially for its usage in the field. But I did not find anything better and do not want to drift away into hair-splitting or a picky assessment.
HGD
Behzad Dastjerdy The important point I would add is that you must quantify the inherent sampling variability in your thin section analyses. It could be several 10's of % (yes, seriously...).
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