A partially protonated form of the mineral umbite has been prepared by ion exchange of K2ZrSi3O9 center dot H2O with acetic acid. The protonated phase, compound 1, is assigned the formula H1.45K0.55ZrSi3O9 center dot 2H(2)O and crystallizes in the space group P2(1)/c with unit cell dimensions of a = 7.1002(2), b = 10.1163(3), c = 13.1742(5), and beta = 91.181(1)degrees. The characteristic building blocks of the acid phase are almost identical to those of the parent compound. The framework is composed of polymeric chains of trisilicate groups linked by zirconium atoms, resulting in zeolite-type channels. When viewed down the a axis, two unique ion-exchange channels can be seen. Site I is marked by a 12-membered ring and contains 2 cations. Site 2, a 16-membered ring contains 4 water molecules. Compound 2, consists of a mixed Sr2+ and K+ phase synthesized from 1 by ion exchange with Sr(NO3)(2). Compound 2 has the formula K0.34Sr0.83ZrSi3O9 center dot 1.8H(2)O and crystallizes in the same space group P2(1)/c. It has cell dimensions of a = 7.1386(3), b = 10.2304(4), c = 13.1522(4), and beta = 90.222(1)degrees. The Sr2+ cations are distributed evenly among the two exchange sites, showing no preference for either cavity. Compound 3 is the fully substituted Sr phase, SrZrSi3O9 center dot 2H(2)O, and retains the same space group as that of the previous two compounds having unit cell dimensions of a = 7.1425(5), b = 10.2108(8), c = 13.0693(6), and beta = 90.283(1)degrees. The strontium cations show a slight affinity for ion-exchange site 2, having a higher occupancy of 0.535, while site 1 is occupied by the remainder of the Sr2+ cations with an occupancy of 0.465. Batch uptake studies demonstrate a selectivity series among alkaline earth cations of Ba2+ > Sr2+ > Ca2+ > Mg2+.