Grandidierite occurs in the outer margin of a biotite-rich pelitic xenolith in norite close to the contact of a tourmaline-bearing granite in the Haddo House norite-gabbro complex. The boron for the grandidierite was derived by metasomatism from the granite. The assemblage is (decreasing order): An_55–51 plagioclase (65%), biotite (10%), spinel (10%), cordierite (5%), corundum (5%), ilmenite, grandidierite (< 1%), rutile, sphene, zircon. X_Mg range is grandidierite (0.80–0.78) > cordierite (0.78) > biotite (c. 0.59) > spinel (0.35–0.27). The grandidierite has little chemical variation: SiO₂, 19.9–20.5%; Al₂O₃, 51.3–51.9%. A compilation of all grandidierite analyses in the literature shows that the only substitutions are Fe²⁺ ⇌ Mg and Al ⇌ Fe³⁺, the X_Mg range being 0.98 to 0.19.