Summary: Johnsomervilleite, Na₁₀Ca₆Mg₁₈(Fe, Mn)₂₅ (PO₄)₃₆ has Z = 3, a 15.00 Å, c 42.75 Å and cell volume 8330 ų and is a new rhombohedral species occurring in two metamorphic segregation pods in Moine gneiss of kyanite-sillimanite grade. Associated minerals are apatite, jahnsite, graftonite, mineral A, phosphosiderite, rockbridgeite, mitridatite, and vivianite. It is dark brown, brittle, and glassy, H = 4 1 2 , D = 3·35 (calc. 3.41) with a perfect basal (?) cleavage. Eight electron probe analyses are given and d spacings for 50 lines. The mineral, which is a Mg-Fe analogue of fillowite, is named after the late Mr J. M. Somerville who presented a specimen of garnet-quartz rock, which contained the new species, to the Royal Scottish Museum.