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

The K-41/K-39 reduced partition function ratios, 10(3)ln beta, of 17 major K-bearing minerals have been calculated using the density functional theory (DFT) method. Their 10(3)ln beta decrease in the order of alunite (KAl3(SO4)(2)(OH)(6)) similar to K-hollandite I (KAlSi3O8) > niter (KNO3) > potassium carbonate (K2CO3) similar to potassium bicarbonate (KHCO3) > muscovite (KAl2(AlSi3O10) (OH)(2)) > potassium hydroxide monohydrate (KOH center dot H2O) similar to hydrated potassium carbonate (K2CO3 similar to 1.5H(2)O) > nepheline (Na3KAl4Si4O16) > potassium hydroxide dihydrate (KOH center dot 2H(2)O) > kalsilite (KAlSiO4) > microcline (KAlSi3O8) similar to phlogopite (KMg3AlSi3O10(OH)(2)) > lepidolite (KLi2AlSi4O10(OH)(2)) > sylvite (KCl) > leucite (KAlSi2O6) > djerfisherite (K6CuFe24S26Cl). The calculated 10(3)ln beta varies from 6.80 parts per thousand in alunite to 2.08 parts per thousand in djerfisherite at 300 K, and from 0.63 parts per thousand to 0.19 parts per thousand at 1000 K. At 1000 K, there is only small variation (< 0.12 parts per thousand) in 10(3) ln alpha(mineral-microcline) of K-41/K-39, defined as 10(3)ln beta(mineral) - 10(3)-ln(beta microcline), for K-bearing silicate minerals except K-hollandite I, indicating no measurable K isotope fractionation among these end-member K-bearing minerals during high-temperature geochemistry processes. However, the K concentration variation in K-feldspar shows a significant effect, as large as 0.21 parts per thousand, on the equilibrium K isotope fractionation between K-feldspar and microcline, which cannot be ignored in high-temperature geochemistry processes. Finally, because KOH center dot H2O and KOH center dot 2H(2)O are enriched in heavy K isotope relative to all calculated silicate minerals except muscovite, we infer that the interaction between water and silicate minerals likely enrichs K-41 in the fluid, which probably explains the relatively higher K-41/K-39 in river and sea water relative to silicate minerals. (C) 2019 Elsevier Ltd. All rights reserved.