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Shand Index (A/CNK & A/NK) Calculator

Calculate the Shand alumina-saturation indices A/CNK and A/NK from major-oxide data on a molar basis, with a neutral peraluminous / metaluminous / peralkaline descriptor.

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How to use this calculator

Major oxides (wt%)

Indices use molar oxide proportions, not weight-percent ratios. Classification labels depend on context and data treatment.

Alumina saturation

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More needed
  • Enter Al₂O₃, CaO, Na₂O and K₂O.
Assumptions & basis
  • Basis: Molar oxide proportions (not weight-percent ratios) (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 Al₂O₃, CaO, Na₂O and K₂O in wt%.
  2. Read A/CNK and A/NK plus the molar oxide proportions used.
  3. Treat the classification descriptor as context-dependent, not definitive.

How it works

The indices are ratios of MOLAR oxide proportions: A/CNK = mol(Al₂O₃) ÷ (mol(CaO) + mol(Na₂O) + mol(K₂O)) and A/NK = mol(Al₂O₃) ÷ (mol(Na₂O) + mol(K₂O)), where each molar amount is the oxide wt% divided by its formula mass. Using moles, not weight percents, is essential — the oxides have very different molar masses.

By convention A/CNK > 1 is peraluminous, A/CNK ≤ 1 with A/NK ≥ 1 is metaluminous, and A/NK < 1 is peralkaline. These labels depend on geological context and data treatment (for instance whether a CaO apatite correction is applied), so the tool reports the numbers and a neutral descriptor rather than asserting a rock type.

Worked example

Al₂O₃ 15, CaO 5, Na₂O 3, K₂O 4 (wt%). Molar: Al₂O₃ 0.14712, CaO 0.08916, Na₂O 0.04840, K₂O 0.04247. A/CNK = 0.14712 ÷ (0.08916 + 0.04840 + 0.04247) = 0.817; A/NK = 0.14712 ÷ (0.04840 + 0.04247) = 1.619 — metaluminous.

Common mistakes

  • Using weight-percent ratios instead of molar proportions.
  • Reading the descriptor as a definitive rock classification.
  • Ignoring that a CaO apatite correction can shift A/CNK for P-rich rocks.

Frequently asked questions

Why molar and not weight percent?

Because the indices compare mole proportions of oxides, and Al₂O₃, CaO, Na₂O and K₂O have very different molar masses. Ratioing weight percents would give the wrong answer.

What do the labels mean?

A/CNK > 1 is peraluminous (excess alumina), metaluminous sits between, and A/NK < 1 is peralkaline (excess alkalis). Interpretation depends on context and data treatment.

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Cite this tool

Zerdly. (2026). Shand Index (A/CNK & A/NK) Calculator. Retrieved from https://www.zerdly.com/tools/shand-index-calculator/

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

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