Reaction of biotite and phlogopite with acidic, CuSO 4 -rich aqueous solutions at 25 degrees + or - 3 degrees C and 1 atm, produced submicroscopic inclusions of a copper-rich, sulfur-absent phase, identified by electron diffraction and analytical electron microscopy as native copper. The copper inclusions lie in the interlayer region of the mica and are associated with copper-enriched, expanded (hydrated) interlayers and potassium depletion. The inclusions, and associated biotite textures and compositions produced in the experiments, are similar to those documented in naturally occurring biotites from rocks associated with porphyry copper deposits (Ilton and Veblen, 1988). The experiments support the suggestion of Ilton and Veblen (1990, 1992) that this mode of copper enrichment in biotite is produced during weathering of rocks with copper sulfide mineralization, such as porphyry copper systems.Electron microprobe analyses of reacted micas show that phlogopite becomes more enriched in copper than biotite but that both phlogopite and biotite become enriched in silica relative to all other cations. A combination of transmission electron microscopy (TEM) observations and electron microprobe analyses indicates that copper is absorbed into the interlayer region of the micas where it exchanges for potassium. We propose that octahedrally coordinated ferrous iron reduces absorbed copper ions to metallic copper. It appears that biotite retains potassium more tenaciously than phlogopite. Higher fluorine concentrations in biotite relative to phlogopite may stabilize potassium in biotite relative to phlogopite. This, in turn, could explain why phlogopite appears to absorb copper more readily than biotite.The results of these experiments indicate that biotite may absorb copper during in situ and heap leach mining, which could result in lowered recoveries. This is relevant to research on in situ leach mining by the U.S. Bureau of Mines, Advanced Mining Division. Biotite in mine tailings and in other types of mine workings could also act as a copper sink, which would lower copper concentrations in acidic mine drainage from some mining regions.