Write a balanced chemical reaction illustrating the substitution mechanism between albite and anorthite in the plagioclase feldspars.
Solution
Albite formula = NaAlSi3O8
Anorthite formula = CaAl2Si2O8
Reaction is: nNaAlSi3O8 + nCaAl2Si2O8 = (NaAlSi3O8 – CaAl2Si2O8)n (repeating units)
Albite + Anorthite = plagioclase feldspars
In case if we substitute 2 Al3+ for Si4+ than it allows Ca2+ to be added Albite-Anorthite series. Further, feldspars are commonly depicted by the formula = XAl(1-2)Si(3-2)O8 .
Where: X = Na / K / Ca.
In case of Na+ and K+ the formula will contain 1Al & 3Si ions and in case of Ca2+ the formula will contain 2Al & 2Si ions for a balanced chemical formula.
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