Zerdly

Feldspar An-Ab-Or Calculator

Calculate feldspar anorthite–albite–orthoclase (An–Ab–Or) endmember proportions from Ca-Na-K cations or oxides, with a ternary plot and FoXX-style notation — a compositional representation, not a mineral identification.

100% FreeRuns in your browserIntermediate
How to use this calculator

Composition

Endmembers use cation proportions and are a composition, not a mineral identification.

An–Ab–Or

Runs entirely in your browser. Your data is not uploaded or stored by Zerdly.

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 Ca, Na and K as cations, or CaO, Na₂O and K₂O as wt% (the tool converts to cations).
  2. Read the An, Ab and Or percentages, the point on the An–Ab–Or ternary and the notation.

How it works

The endmembers are the cation proportions normalised to 100%: An = Ca/(Ca+Na+K), Ab = Na/(Ca+Na+K), Or = K/(Ca+Na+K). Cation amounts are used, not oxide weight percents — from oxides the tool first converts each to cations (Na₂O gives two Na per formula, CaO one Ca).

The result is a composition plotted on the An–Ab–Or ternary; it represents where the analysis sits between the three endmembers, not a definitive mineral name. Zero sums and non-numeric entries are handled explicitly.

Worked example

Ca 0.4, Na 0.5, K 0.1 cations. An = 0.4/1.0 = 40%, Ab = 0.5/1.0 = 50%, Or = 0.1/1.0 = 10% — an An40 Ab50 Or10 feldspar (andesine–oligoclase field).

Common mistakes

  • Using oxide wt% directly instead of cation proportions.
  • Forgetting Na₂O and K₂O each contribute two cations per formula.
  • Reading the endmember composition as a mineral identification.

Frequently asked questions

Can I enter oxides?

Yes — switch to oxide mode and enter CaO, Na₂O and K₂O in wt%; the tool converts them to Ca, Na and K cations before computing the endmembers.

Is this a mineral identification?

No. An–Ab–Or is a compositional representation of where the feldspar sits between the three endmembers, not a species determination.

Related tools

Explore more in Geology, Geotechnical & Ground Engineering.

Cite this tool

Zerdly. (2026). Feldspar An-Ab-Or Calculator. Retrieved from https://www.zerdly.com/tools/feldspar-an-ab-or-calculator/

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

All calculations run in your browser. Your inputs are never saved or transmitted.