Another embodiment of the invention is a crystalline composition having the empirical formula expressed in molar ratios: xR:M1-y M'y A2???0.2 :zH2 O where y is greater than 0 to about 0.5 and M' is a metal that meets the criteria: (a) occurs in sulfide or selenide minerals of hydrothermal origin; (b) exhibits stable aqueous thiometallate or selenometallate chemistry or forms stable bisulfide or bi