The structure of an authigenic maximum microcline from the Pontiskalk limestone, Switzerland, has been refined to an R value of 8.5% by a full-matrix least-squares program using 797 observed and 207 unobserved reflections. The ordering of tetrahedral Si,Al atoms is nearly complete and there is no significant structural difference from an igneous maximum microcline. The crystal is considered to have grown very slowly at a low temperature within the stability field of the fully ordered phase.