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Shand (1927) grouped igneous rock based on the total molar alkali verse alumina content. Which condition will be suitable in case of Pre-aluminous ?

  • A.Al\(_{2}\)O\(_{3}\) < (Na\(_{2}\)O + K\(_{2}\)O)
  • B.Al\(_{2}\)O\(_{3}\) > (CaO + Na\(_{2}\)O + K\(_{2}\)O)
  • C.Al\(_{2}\)O\(_{3}\) < (CaO + Na\(_{2}\)O + K\(_{2}\)O) but Al\(_{2}\)O\(_{3}\) > (Na\(_{2}\)O + K\(_{2}\)O)
  • D.Al\(_{2}\)O\(_{3}\) = (Na\(_{2}\)O + K\(_{2}\)O)

Answer: B

The condition for peraluminous ('Pre-aluminous') rocks is Al2O3 > (CaO + Na2O + K2O), in molar proportions.

Shand's alumina saturation scheme compares molar alumina with the alkalis and lime:

  1. Peraluminous: Al2O3 greater than CaO + Na2O + K2O; the excess alumina appears as muscovite, cordierite or garnet, as in S-type granites.
  2. Metaluminous: Al2O3 less than CaO + Na2O + K2O but greater than Na2O + K2O; the third option.
  3. Peralkaline: Al2O3 less than Na2O + K2O; the first option, with sodic amphiboles or pyroxenes.

Equality of alumina and alkalis is only the boundary between metaluminous and peralkaline.

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