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Olivine Fo-Fa Calculator

Calculate olivine forsterite–fayalite (Fo–Fa) endmembers and Mg# from Mg-Fe cations or MgO/FeO, with FoXX notation — using a ferrous-iron basis you can state explicitly.

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How to use this calculator

Composition

Uses ferrous-iron basis; Fo% equals Mg#. If only total Fe is known, state the assumption.

Fo–Fa

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More needed
  • Enter the component values.
Assumptions & basis
  • Basis: Cation proportions (a composition, not an identification) (derived)

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

  1. Enter Mg and Fe²⁺ as cations, or MgO and FeO as wt% (converted to cations).
  2. Read Fo% and Fa%, the Mg# (which equals Fo%) and the composition bar.

How it works

Forsterite and fayalite are the Mg and Fe endmembers of olivine: Fo = Mg/(Mg+Fe²⁺) × 100 and Fa = Fe²⁺/(Mg+Fe²⁺) × 100, on a cation basis. Because it is a two-component Mg–Fe ratio, Fo% is numerically the same as the Mg number (Mg#).

Olivine iron is essentially ferrous, so the calculation uses Fe²⁺. If only total iron is available, treating it all as Fe²⁺ is an assumption worth stating — it will differ if any iron is ferric.

Worked example

Mg 1.8, Fe 0.2 cations. Fo = 1.8/(1.8+0.2) × 100 = 90%, Fa = 10% — a forsteritic Fo90 olivine, Mg# 90.

Common mistakes

  • Ratioing MgO and FeO weight percents directly instead of converting to cations.
  • Using total iron as Fe²⁺ without stating it.
  • Treating Fo% and Mg# as different numbers — for olivine they are the same.

Frequently asked questions

Is Fo% the same as Mg#?

For olivine, yes — both are Mg/(Mg+Fe²⁺) × 100. An Fo90 olivine has Mg# 90.

Which iron should I use?

Ferrous iron (Fe²⁺ / FeO). Olivine iron is essentially ferrous; if you only have total Fe, state that you are treating it as Fe²⁺.

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Cite this tool

Zerdly. (2026). Olivine Fo-Fa Calculator. Retrieved from https://www.zerdly.com/tools/olivine-fo-fa-calculator/

Tip: Enter any known values to calculate the remaining results.

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