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2 years ago in Chemistry , Geochemistry By Renu

How can I convert wt% of oxide to apfu (atoms per formula unit) without Li2O and H2O?

I'm working on a dataset of granite whole-rock chemistry where Li and H were not analyzed. I need to normalize cations to a standard formula (e.g., for biotite or muscovite) for plotting and comparison. Is there a reliable stoichiometric constraint or normalization procedure I can use to approximate the Li and H sites, or must I simply exclude minerals requiring those elements?

 

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By Shreya K Answered 1 year ago

This is a common lab dilemma. For phases like micas or amphiboles, you cannot accurately calculate the formula without H2O, as it's a major structural component. For Li, in minerals like amblygonite, it's critical. However, for many rock-forming silicates in granites, I use a two-step approach. First, normalize your major cations to a fixed oxygen basis (e.g., 22 for micas). Then, for the A-site (where K, Na, Ca, and Li go), if your (Na+K) sum is significantly less than the ideal site occupancy, you can note that a deficit exists, possibly filled by Li, but you cannot quantify it. I would recommend clearly stating the omission in your methods and focusing your interpretation on the major cations you have.

 

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