Dear Akram,

            There are several excel sheets freely available in the net. I also suggest you refer the third edition of 'An Introduction to the Rock-Forming Minerals' by Deer, Howie & Zussman or the individual volume pertaining to micas or sheet silicates. Hope this helps.

Dear Akram,

this apparently trivial problem is indeed a bit more complex, inasmuch as in biotite, as in many other minerals, we reconstruct the crystal chemical formula starting from an incomplete analytical dataset, i.e., we usually don't analyze hydrogen. The correct question should be "which negative charge I have in biotite anions?" Usually we don't know, as we cannot quantify all the oxy substitutions in biotite. If you don't analyze hydrogen, 22 or 24 anions are formally the same thing, it's just the trick of  a different choice, but make clear how many negative charges you consider. Just specify somewhere what you did, and it should be enough. For details and different normalization approaches, you can also look at Dymek 1983, Am. Mineral., but keep in mind that neither in that paper oxy subsitutions are taken into account, they are ony cited as "disturbs" in the calculations. Hope that this helps, without adding further confusion

Dear Akram,

As generally accepted method, the structural formula or unit-cell compositions of biotites as well as other micas, illite, and smectite were calculated on the basis of 22 negative charges (corresponding to 10 oxide and two hydroxide ions or 11 oxygen atoms (e.g., Weaver and Pollard, 1973). You may easily check it a lot of papers published in high impact journals. Good luck.

Dear Jayabalan Manickam , Sandro Meli  and Omer Bozkaya 

Thanks for the answers.

The best

Akram