Biotite and amphibole containing up to 4.6 and 5.1 wt% Cl, respectively, are found in gneisses from Black Rock Forest, located within the Hudson Highlands of New York. The area was metamorphosed to granulite facies during the Grenvillian orogeny at similar to 1100 Ma. Small, subeconomic magnetite deposits coexist with the gneisses.Cl contents in biotite range from 0.1 to 4.6 wt%. Some individual biotite grains are spectacularly zoned, with Cl contents varying from 1.0 to 4.6 wt% within <20 mu m. In zoned biotite, an increase in Cl content correlates with increasing Fe and decreasing Mg, Ti, and F. When data from five specimens are compared, however, Cl contents in biotite show only two strong correlations: Cl contents correlate positively with Fe (r(2) = 0.75) and negatively with Ti (r(2) = 0.84). Cl and Mg are uncorrelated for the complete dataset but show strong correlation within individually zoned grains (r(2) = 0.74). F contents show only one correlation, a positive one with Mg (r(2) = 0.62). The structural formula for the most Cl-rich biotite analyzed is (K2.00Na0.02)(Mg1.94Fe3.90Ti0.07Al0.04) (Si5.85Al2.15)O-20(OH2.46F0.25Cl1.29), as normalized to 24 (O + OH + F + Cl). This biotite is one of the most Cl rich ever reported.Cl contents in amphibole range from 0.2 to 5.1 wt%. Cl contents correlate positively with Fe2+/(Fe2+ + Mg) (r(2) = 0.84) and K (r(2) = 0.70) and negatively with Si/(Si + Al-[4]) (r(2) = 0.49) and Ti (r(2) = 0.76). F contents are uncorrelated with any of the other elements analyzed. The structural formula for the most Cl-rich amphibole, a hastingsite, is (K0.57Na0.31)(Ca1.90Na0.10)(Fe2.692+Mg0.90Mn0.03Ti0.06Fe1.023+Al0.29)(Si5.80Al2.20)O-22(OH0.55F0.07Cl1.38), as normalized to 13 octahedral and tetrahedral cations.In rocks with coexisting amphibole and biotite, Cl partitions preferentially into the amphibole and F partitions preferentially into the biotite. Infiltration of the gneisses by fluids rich in Fe and Cl (perhaps as an FeCl2 compound) could explain the formation of magnetite bodies as well as the Fe- and Cl-enriched phases found in and around Black Rock Forest.