First-principles investigation of equilibrium K isotope fractionation among K-bearing minerals

The 41K/39K reduced partition function ratios, 103lnβ, of 17 major K-bearing minerals have been calculated using the density functional theory (DFT) method. Their 103lnβ decrease in the order of alunite (KAl3(SO4)2(OH)6) ∼ K-hollandite I (KAlSi3O8) > niter (KNO3) > potassium carbonate (K2CO3) ∼ potassium bicarbonate (KHCO3) > muscovite (KAl2(AlSi3O10)(OH)2) > potassium hydroxide monohydrate (KOH·H2O) ∼ hydrated potassium carbonate (K2CO3·1.5H2O) > nepheline (Na3KAl4Si4O16) > potassium hydroxide dihydrate (KOH·2H2O) > kalsilite (KAlSiO4) > microcline (KAlSi3O8) ∼ phlogopite (KMg3AlSi3O10(OH)2) > lepidolite (KLi2AlSi4O10(OH)2) > sylvite (KCl) > leucite (KAlSi2O6) > djerfisherite (K6CuFe24S26Cl). The calculated 103lnβ varies from 6.80‰ in alunite to 2.08‰ in djerfisherite at 300 K, and from 0.63‰ to 0.19‰ at 1000 K. At 1000 K, there is only small variation (