I obtained a chemical composition of a plagioclase grain in weight percentage using SEM-EDS technique. The recalculated chemical formula of the grain is as follows using an MSExcel worksheet based programme.
Na0.5Ca0.2Al1.1Si2.3O8
Here the charge balance is problematic. What could I have possibly missed here?
Thanks in advance
Hi Tharanga,
I am a little bit curious on the analysis conditions (kV, probe current, focus size, analysis time, ...). I have made the experience that especially Na is mobile under an electron beam. If your beam was too strong you might have a far too low Na signal.
Besides that, the formula you posted seems to miss something. Do you have K, Ba or Sr in you spectrum?
I can offer you to fill your data in my feldspar sheet and if the results are different.
yes, it looks like something is missing, can you post your EDS data and maybe the spectrum?
The total of Ca+Na+K should equal to 1 in the formula, SI+Al should equal to 4 also
other wise the analysis is incomplete. Calculation should be on the base of 8 Oxygen
I agree with Florian Ströbele, that Na is mobile under influence of the electron beam. With high Na-content, actually one should slightly move the beam within the grain to analyse a fresh spot everytime. Having your beam during the whole analysis on the same spot may lead to a too low Na-content, because Na moves away from the beam..
you are not missing any copound , but think that you add NaO which should be not found
Dear Tharanaga: The main problem I see in your albite formula, besides the obvious difficulty of analyzing Na, is that the total of tetrahedral cations is not equal to 4, but to 3.4. This is NOT possible, as would not be representative of a tectosilicate feldspar structure, totally polimerized forming a 3-D network of tetrahedra sharing all four corners. Surely, the large cation site in the structure -with a high coordination number, can also be occupied by percentages of K, Ba, Sr, and other alkali or alkali-earth cations. But the REAL problem you have is that the total of tetrahedrically coordinated cations is not adding up not even near 4. With regards.
We should also know if oxygen was calculated or measured? Another point being there are a number of other minerals with these elements, eg, scapolite ...
Dear Tharanga. It would be better to use (if you have very good quality polished thin sections of your samples) electron microprobe analysis (EPMA). Best wishes, Anicet.
Dear Tharanga,
Here is a copy of a paper that I authored with chemical formula of albite based on EPMA analyses from parent rock minerals in lateritic weathering profiles of Central Africa. Kind regards, Anicet
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