The potassium content of muscovite (K => range 10 - 12 wt%) makes the difference.

Pyrophyllite does not contain any potassium!

Best regards,

Guenter Grundmann

Muscovite contains potassium, which is absent in pyrophyllite. Since pyrophyllite is a hydrous aluminosilicate, it contains high silica (~65%) compared to muscovite (45% silica).

Thanks Guenter and Ashutosh.

Dear Mrs. Chaudhuri:

In principle the key element has already been mentioned. It is potassium. My question is simply related to the type of clay mineral assemblage and how did you take your decision in favor of pyrophyllite. If you assume pyrophyllite based upon your analysis your chemical composition should be Al2Si4O10(OH)2 , if you take into consideration one of the three polymorphs of the kaolinite-group phyllosilicates it is Al2Si2O5(OH)4; you can extend this issue also to halloysite and metahalloysite which have in its de-hydrated modification the same chemical composition Al2Si2O5(OH)4 . All these clay minerals may be encountered in so-called high-sulfidation epithermal mineralizations associated which each other and also with muscovite, where they can also appear as 7-10 Å mixed-layers with end-memberoriginating from each clay mineral group mentioned in this text. Therefore be assured that these phyllosilicates are checked as to their attribution to one or the other clay mineral group by textural XRD analysis.

H.G.Dill

Dear Prof. Dill,

Thank you for your thoughtful suggestions. I have some high-Al pelites which consists of Kyanite and muscovite. So, initially I thought all the while micaceous minerals are muscovite. But during EPMA study, I detected some clay minerals adjacent to kyanite blades. I was wondering about the mineral (whether muscovite or pyrophyllite) that has participated in kyanite formation.

I had a suspicion about pyrophyllite as my assemblage doesn't contain any K-bearing mineral (except mus). If muscovite participates in kyanite formation, then some K-bearing mineral should have been produced as kyanite doesn't accommodate K. Thus I was trying to find out if my rock has pyrophyllite where the problem of potash balance does not persist. I have not considered the other possible clay minerals like you suggested. I may run my samples under Raman spectroscope to check what kind of clay they are.

Thanks again.

Dear Mrs. Chaudhuri,

the presence of kyanite signals the rock to be a meta-pelite where pyrophyllite may come into existence during retrograde metamorphic processes (low grade stage), but also kaolinite-group minerals. Therefore micro-Raman and / or XRD are suggested.

With kind regards

H.G.Dill

Thanks Professor Dill.

Mrs. Chaudhuri,

You have detailed answer from Prof H Dill. If you are sure that your mineral is either muscovite or pyrophyllite, the EPMA analysis can be readily used, because the former has lots of K2O and the latter has none or little of it. But there are possibilities of other minerals, also, as Prof Dill advised. You may have to go in greater details than simply assuming you may possibly be dealing with pyrophyllite.

If I could suggest that pyrophyllite (word means "leaves on fire") has this flame or star-like appearance on thin section that you can't miss (usually).

Thanks Michael. I'm not well acquainted with the appearence of pyrophyllite under microscope. I may have missed it.

attached picture of pyrophyllite replacing muscovite