A monoclinic analogue of the mineral hydroxylapatite was found in altered leucogabbro making up one of the intrusive units in the Mesoproterozoic Seagull pluton in northwestern Ontario, Canada. The mineral is part of a replacement assemblage developed metasomatically after igneous plagioclase. It occurs as masses of coalescent spherulites < 30 μm in diameter, and is paragenetically associated with prehnite, hibschite, titanite, rutile and an unidentified Ca silicate. The new mineral is white and chalky in appearance, has a Mohs hardness of 5 and a measured density of 3.07(2) g/cm3 (Dcalc 3.13 g/cm3), and shows little deviation from a uniaxial optical behavior (α 1.632, γ 1.649). Compositionally, it is an intermediate member of the ternary system Ca5(PO4)3(OH) - Ca5(PO4)3Cl - Na3Ca2(SO4)3(OH). The average content of the latter two end-members is ca. 20 mol. % each. The combination of high Na and S contents (up to 0.63 and 0.71 apfu, respectively) is unparalleled by any previously reported natural composition, and indicates significant solubility of these elements in natural apatites even at low crystallization temperatures (< 400 °C). The monoclinic symmetry of this mineral, determined from microbeam X-ray diffraction patterns, distinguishes it from hydroxylapatite. By analogy with synthetic Ca5(PO4)3OH, its space group is P21/b [a 9.445(2) Å, b 18.853(4) Å, c 6.8783(6) Å, γ 120.00(2)°], and the deviation of symmetry from the archetypal (space group P63/m, a ∼9.4 Å, c ∼6.9 Å) probably results from ordering of (OH) anions in [00z] anionic columns and consequent doubling of periodicity along . In keeping with the nomenclature of apatites, this new mineral was named clinohydroxylapatite.