Boromullite is a new mineral corresponding to a 1:1 polysome composed of Al5BO9 and Al2SiO5 modules. Electron-microprobe analysis of the holotype prism is SiO2 19.01(1.12), TiO2 0.01(0.02), B2O3 6.52(0.75), Al2O3 74.10(0.95), MgO 0.07(0.03), CaO 0.00(0.02), MnO 0.01(0.04), FeO 0.40(0.08), Sum 100.12 wt.%, which gives Mg0.01Fe0.03Al8.88Si1.93B1.14O18.94 (normalised to 12 cations), ideally Al9BSi2O19. Overall, in the type specimen, it ranges in composition from Mg0.01Fe0.03Al8.72Si2.44B0.80O19.20 to Mg0.01Fe0.03Al9.22Si1.38B1.35O18.67. Single-crystal X-ray diffraction gives orthorhombic symmetry, Cmc21, a 5.7168(19) Å, b 15.023(5) Å, c 7.675(3) Å, V 659.2(7) Å3, calculated density 3.081 g/cm3, Z = 2. The refined structure model indicates two superimposed modules present in equal proportions in the holotype prism. Module 1 has the topology and stoichiometry of sillimanite and carries all the Si, whereas module 2 is a type of mullite defect structure in which Si is replaced by B in triangular coordination and by Al in tetrahedral coordination, i.e., Al5BO9. The strongest lines in the powder pattern [d in Å, (Imeas.), (hkl)] are 5.37(50) (021), 3.38(100) (022, 041), 2.67 (60) (042), 2.51(60) (221, 023), 2.19(80) (222), 2.11(50) (043), 1.512(80) (263). Boromullite is colourless and transparent, biaxial (+), nx 1.627(1), ny 1.634(1), nz 1.649(1) (589 nm). 2Vz (meas) = 57(2)−, 2Vz (calc) = 69(12) −. In the type specimen boromullite tends to form prisms or bundles of prisms up to 0.4 mm long, typically as fringes or overgrowths on aggregates of sillimanite or as narrow overgrowths around embayed werdingite prisms. In other samples boromullite and sillimanite are intergrown on a fine scale (from <1 μm to >10 μm). Sekaninaite-cordierite, potassium feldspar, biotite, werdingite and its Fe-dominant analogue, hercynite, and ilmenite are other commonly associated minerals, whereas ominelite-grandidierite, plagioclase, andalusite, and tourmaline are much subordinate. The most widespread accessories are monazite-(Ce), an apatite-group mineral and zircon. Boromullite formed during anatexis of B-rich pelitic rocks under granulite facies conditions (810 °C ≈ T ≥ 775–785 °C, P = 3.3–4 kbar), possibly due to a shift in bulk composition to lower SiO2 and B2O3 contents associated with melt extraction. The assemblage boromullite + cordierite + sillimanite lies at lower SiO2 and B2O3 contents than the assemblage werdingite + cordierite + sillimanite and thus a decrease in SiO2 and B2O3 leads to the replacement of werdingite by boromullite, consistent with textural relations.