Igneous and hydrothermal biotite in the Santa Rita stock, which hosts the Santa Rita porphyry copper deposit, has been analyzed by electron-microprobe techniques for K, Na, Ca, Ba, Mg, Mn, Ti, Al, Si, Cl, and F; biotite separates were analyzed for ferrous and ferric iron by wet-chemical analysis and atomic absorption. Igneous biotite occurs as phenocrysts and microphenocrysts. Hydrothermal biotite occurs as secondary disseminated flakes, as aggregates of flakes resulting from recrystallization of igneous biotite or replacement of igneous hornblende, and as vein fillings. Included in the analyses are biotite in the least-altered part of the Santa Rita stock and in potassic, argillic, phyllic, and propylitic alteration associated with the stock. Of the 12 elements analytically sought, Ba, Fe, Mg, Ti, Al, Si, Cl, and F show the greatest variations.Systematic variations in the composition of biotite are apparent in the Santa Rita stock and in each type of alteration except phyllic. Mole fraction phlogopite, F, and Si increase progressively from igneous to secondary to vein biotite; the reverse relationship holds for Ti, Cl, and Al. Mole fraction phlogopite correlates positively with F and Si and negatively with Ti, Cl, and Al. These correlations, with the exception of that involving Ti, are well developed in the phyllic alteration. In phyllic alteration, however, the systematic relationship between the different varieties of biotite and their composition is obliterated. In all types of alteration, igneous biotite is distinguished from hydrothermal biotite by its higher Ba content. The Fe (super +2) /Fe (super +3) ratio increases progressively from igneous to secondary to vein biotite in the least-altered and potassically altered parts of the stock. The composition of overgrowths and protruding parts of igneous phenocrysts is intermediate between that of phenocryst and secondary biotite. The Al content of both igneous and hydrothermal biotite associated with phyllic alteration is notably higher than in biotite associated with other types of alteration; also, in general, the higher Al content correlates with a lower phlogopite content.Interpretation of the compositions of the several varieties of biotite in the Santa Rita porphyry copper deposit with various experimental and theoretical investigations and thermodynamic relationships suggests that an aqueous fluid phase exsolved from a partially crystallized melt at a T [asymp] 745 degrees C and a P [asymp] 1 kb. Subsequent hydrofracturing of the solidified part of the stock resulted in a lower T and P environment of potassic alteration which is suggested to range from about 670 degrees to 580 degrees C and 220 to 20 bars.The variable but systematically related content of Fe, Mg, Ti, and Al in relic biotite in the phyllic and argillic alteration of the Santa Rita deposit have resulted mainly from reequilibration with a solution that had variable ratios of Fe, Mg, and Ti to pH; this hydrothermal fluid encroached upon the potassic alteration zone at a temperature estimated to have ranged from 310 degrees to 370 degrees C. A postulated trend of biotite compositions across the phyllic-potassic transition zone of alteration and into the argillic zone indicates a progressive increase in pH and in a Mg (super +2) /a Fe (super +2) of the aqueous phase in that direction.