The Datolite Group (which is also known as the Datolite/Gadolinite Group after its two most common members) is composed of similarly structured nesosilicates. Its members contain alternating borate tetrahedrons, B(O, OH)4, and silicate tetrahedrons, SiO4, that are connected to each other in a sheet like structure. Between the tetrahedral sheets and linking the tetrahedral sheets are metal cations in 8 coordination. Dispite the layered structure, cleavage is all but completely absent. Some members have beryllium instead of boron in the non-silicate tetrahedrons. Several members have rare earth metals as part of their compositions. The general formula for the Datolite Group is approximately: X(B, Be)SiO4(OH, O). The X ion can be either calcium, magnesium, cerium, lanthanum, iron, tin, niobium, yttrium, ytterbium and/or erbium.