The occurrence of kerolite in association with various secondary Ca-Mg carbonate mineral deposits (speleothems) was identified in basaltic sea caves on the island of Kauai, Hawaii. Kerolite is the dominant clay mineral in the deposits. X-ray diffraction (XRD) peaks of the kerolite are characteristically broadened indicating its extremely poor crystallinity. Few changes were observed in the XRD patterns of this kerolite when it was subjected to various humidity, temperature and ethylene-glycol treatments. The crystals appear as flaky masses with irregular or jagged edges in scanning (SEM) and transmission electron microscopy (TEM). Electron probe and energy dispersive X-ray (EDX) microanalysis show that the clay material is dominated by Mg-Si-O, with minor amounts of Al and Ca in some samples. The chemical composition, thermal analysis and TEM observations suggest that smaller amounts of an amorphous serpentine-like phase are mixed with the kerolite. Kerolite is often the only mineral associated with poorly mineralized, actively-growing microbial mats in these caves and it is common in completely lithified microbial mats. The latter commonly have microstromatolitic structures with kerolite as a dominant phase. These features suggest that kerolite formation is at least in part a result of microbial activity. The abundant extracellular polymers of the mat-forming bacteria bind and concentrate ions (Mg2+, silica) from solution and serve as nucleation sites for kerolite precipitation. Conditions within the mats also probably lead to formation of Mg-Si-gels, amorphous Mg-silicate precursors and ultimately kerolite. Evaporation of the cave solutions may also contribute to kerolite formation.