Kyrian

You will certainly want to look at:

Nitrogen isotope fractionation during ammonium exchange reactions with clay minerals, by Karamanos R.E. and Rennie D.A. (1978) in Canadian Journal of
Soil Science, 58, pp 53-60.

They talk about two stages of Ca (or K) replacement by NH4+, with a final "equilibrium" fractionation of 7 to 11 per mil.

Best wishes

Tim Heaton

NERC Isotope Geosciences Laboratory
Keyworth
Nottingham NG12 5GG UK

E-mail: [log in to unmask]