Tobermorite

Tobermorite is a calcium silicate hydrate mineral with the general composition Ca5Si6O16(OH)2·nH2O, where the hydration state varies. The best-known crystalline forms are the 11-angstrom and 14-angstrom polytypes, which differ in the stacking of silicate layers. The name honors Tobermory (Tobermore) on the Isle of Mull, Scotland, where the mineral was first described.

In nature, tobermorite forms during hydrothermal alteration of calcium silicate rocks and minerals, typically in low-grade metamorphic settings or in silica-rich fluids reacting with volcanic materials. It is commonly found with other calcium silicate hydrates and zeolites and may occur as crystals or microcrystalline masses. Tobermorite is also encountered in engineered materials, notably in hydrated Portland cement when subjected to elevated temperatures, where it can influence material properties. The 11 Å variety is especially relevant to cement chemistry and some ancient concretes.

Significance lies in both geology and materials science. In historical contexts, tobermorite is associated with the durability of Roman or ancient concretes, where crystalline calcium silicate hydrates contributed to longevity. In modern research, synthetic tobermorite is studied for its potential to strengthen cementitious systems and to immobilize certain radionuclides, owing to its layered silicate structure and ion-exchange capabilities.

Physical characteristics vary with composition and polytype; tobermorite typically appears as colorless to pale crystals or as part of a microcrystalline matrix. Its occurrence ranges from natural hydrothermal alteration zones to laboratory-synthesized phases used in cement science and waste immobilization research.