| Abstract
| - A partially protonated form of the mineral umbite has been prepared by ion exchange of K2ZrSi3O9·H2Owith acetic acid. The protonated phase, compound 1, is assigned the formula H1.45K0.55ZrSi3O9·2H2O andcrystallizes in the space group P21/c with unit cell dimensions of a = 7.1002(2), b = 10.1163(3), c =13.1742(5), and β = 91.181(1)°. The characteristic building blocks of the acid phase are almost identical tothose of the parent compound. The framework is composed of polymeric chains of trisilicate groups linkedby zirconium atoms, resulting in zeolite-type channels. When viewed down the a axis, two unique ion-exchange channels can be seen. Site 1 is marked by a 12-membered ring and contains 2 cations. Site 2, a16-membered ring, contains 4 water molecules. Compound 2, consists of a mixed Sr2+ and K+ phase synthesizedfrom 1 by ion exchange with Sr(NO3)2. Compound 2 has the formula K0.34Sr0.83ZrSi3O9·1.8H2O and crystallizesin the same space group P21/c. It has cell dimensions of a = 7.1386(3), b = 10.2304(4), c = 13.1522(4), andβ = 90.222(1)°. The Sr2+ cations are distributed evenly among the two exchange sites, showing no preferencefor either cavity. Compound 3 is the fully substituted Sr phase, SrZrSi3O9·2H2O, and retains the same spacegroup 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 β = 90.283(1)°. 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 anoccupancy of 0.465. Batch uptake studies demonstrate a selectivity series among alkaline earth cations ofBa2+> Sr2+> Ca2+> Mg2+.
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