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.
ASJC Scopus subject areas
- General Earth and Planetary Sciences