Superparamagnetism in volcanic glasses of the KBS Tuff: Transmission electron microscopy and magnetic behavior

Volcanic glass separates (colorless to dark brown) from the KBS tuff of northern Kenya have been studied with a combination of transmission electron microscopy and low‐temperature ac susceptibility and dc magnetization experiments. The darker of these glasses exhibit classic superparamagnetic behavior, the origin of which lies in a spatially‐uniform precipitate of magnetite, which is present as ∼1% by weight in glass shards with the highest susceptibility. In any given glass separate (obtained by magnetic separation) this precipitate has a surprisingly narrow size distribution. A theory for the origin of the precipitate is nucleatation and growth in quenched glasses at temperatures of ∼1000‐1300°K; an experiment demonstrates the feasibility of this idea. These glasses provide us with a sample for experimental investigations of physical properties of naturally‐occurring dispersed magnetic phases at the lower limit of physical dimension that can be attained in the crystalline state.