Systematic variations in sericite composition are present in each of the areas studied. In the Roosevelt geothermal system, Si increases and tetrahedral Al decreases with increasing depth in the system. Sericites from Southwest Tintic increase in Si and decrease in interlayer cations from biotite through quartz-sericite to chlorite-epidote zones. Compositional variability is less at Santa Rita than the two other areas, and interlayer site occupancy is greater than 0.9, but sericite with epidote and chlorite contains more Si than sericite with biotite and/or clay. Minor elements in sericite also show compositional trends. Sodium occupies from 0 to 0.1 of the interlayer sites and increases from the chlorite-epidote to quartz-sericite to biotite zones in porphyry copper systems. Sericite coexisting with biotite contains significantly more Ti than sericite in other assemblages. Fluorine content of sericites ranges up to 0.14 ions per formula unit and correlates with Mg.--Modified journal abstract.