Tobermorite Group

Tobermorite group compounds are well known in the cement industry. They form when cement reacts with water and play an important role in the binding process.
The structure contains composite layers consisting of infinite sheets of (CaO7) polyhedra, to both sides of which wollastonite-type dreier single chains of (SiO4) tetrahedra are attached, running along the b-axis. Those layers are stacked along the c axis and linked by Si-O-Si bonds between the single chains in adjacent layers, forming dreier double chains whose individual strands belong to different layers. The open channels running along the b axis between the layers are occupied either by water molecules, resulting in the formula Ca4(H2Si6O17)·5H2O (anomalous tobermorite) or by both water molecules and Ca2+ ions, resulting in the formula Ca4.5(HSi6O17)·5H2O (normal tobermorite). Upon heating, the water molecules are lost, and the structure is rearranged in order to fulfill the coordination requirements of the interlayer calcium ions. The structure of clinotobermorite, which also shrinks upon heating, is very closely related to that of normal tobermorite (Merlino et al., 2001).

A new nomenclature scheme for minerals in the Tobermorite Supergroup was approved by the IMA in 2014.

Compare also the related xonotlite.
The members of the Tobermorite group differs in their thermal behaviour which can be ‘‘normal’’ (the phase shrinks to a 9 Å phase at 300°C) or ‘‘anomalous’’ (the phase preserves its 11 Å basal spacing at 300°C).