Mineral Formula / APFU Calculator
Calculate atoms per formula unit (APFU) from an oxide wt% analysis on a chosen oxygen basis, with mineral presets that set the basis — a transparent, step-by-step recalculation rather than a black box.
Oxide analysis (wt%)
| Oxide | wt% | |
|---|---|---|
Generic oxygen-basis APFU. Formula recalculation is convention-dependent; iron valence follows the oxide you enter (no automatic Fe³⁺ solution).
Atoms per formula unit
Runs entirely in your browser. Your assay data is not uploaded or stored by Zerdly.
- Enter at least one valid oxide and wt%.
Assumptions & basis
- Iron valence: Taken from the input oxide (FeO vs Fe₂O₃); not split automatically (user)
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter each oxide (e.g. SiO2, MgO, FeO) with its wt%.
- Choose a mineral preset (which fixes the oxygen basis) or set a custom oxygen count.
- Read the atoms per formula unit for each cation and the total cations.
How it works
Each oxide's mole amount is its wt% divided by its formula mass. Multiplying by the cations and oxygens per formula gives cation and oxygen amounts. Normalising the cations to a fixed number of oxygens per formula unit — 4 for olivine, 6 for pyroxene, 8 for feldspar, and so on — converts the analysis to atoms per formula unit (APFU).
Mineral-formula recalculation is convention-dependent, so this tool implements the transparent generic oxygen-basis method and shows every intermediate. Iron valence follows the oxide you enter (FeO vs Fe₂O₃); it does not apply a universal Fe³⁺ estimation, because that assumption differs between minerals.
Worked example
Forsteritic olivine on a 4-oxygen basis. An olivine of SiO₂ 40.9, MgO 49.4, FeO 9.6 wt% normalised to 4 oxygens gives roughly Si 1.00, Mg 1.79, Fe 0.20 — about 3 cations per formula unit, consistent with (Mg,Fe)₂SiO₄.
Common mistakes
- Using the wrong oxygen basis for the mineral (e.g. 4 vs 6).
- Expecting an automatic Fe²⁺/Fe³⁺ split from total iron.
- Reading APFU totals without checking they match the mineral's site occupancy.
Frequently asked questions
Why normalise to oxygens?
Mineral formulae are defined per a fixed number of oxygens (or cations). Normalising the cation amounts to that oxygen basis yields atoms per formula unit, the standard mineral-chemistry output.
How is iron treated?
By the oxide you enter. FeO is ferrous, Fe₂O₃ ferric. The tool does not estimate Fe³⁺ from stoichiometry, since that convention varies by mineral.
Related tools
- Oxide Moles & Cations Calculator
- Feldspar An-Ab-Or Calculator
- Olivine Fo-Fa Calculator
- Assay Unit Converter
- Elements to Oxide Calculator
- Major Oxide Normalization Calculator
Explore more in Geology, Geotechnical & Ground Engineering.
Cite this tool
Zerdly. (2026). Mineral Formula / APFU Calculator. Retrieved from https://www.zerdly.com/tools/mineral-formula-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.