Kerimasite, ideally Ca₃Zr₂(Fe³⁺ ₂Si)O₁₂, is a new calcium zirconium silicate-ferrite member of the garnet group from the extinct nephelinitic volcano Kerimasi and surrounding explosion craters in northern Tanzania. The mineral occurs as subhedral crystals up to 100 μm in size in calcite carbonatites, and as euhedral to subhedral crystals up to 180 μm in size in carbonatite eluvium. Kerimasite is light to dark-brown in colour and transparent with a vitreous lustre. No cleavage or parting was observed and the mineral is brittle. The calculated density is 4.105(1) g/cm³. The micro-indentation, VHN ₂₅, ranges from 1168 to 1288 kg/mm². Kerimasite is isotropic with n = 1.945(5). The average chemical formula of the mineral derived from electron microprobe analyses (sample K 94-25) and calculated for O = 12 and all Fe as Fe₂O₃ is (Ca_3.00Mn _0.01Ce_0.01Nd_0.01)_Σ3.03(Zr _1.72Nb_0.14Ti_0.08Mg_0.02Y _0.02)_Σ1.98(Fe³⁺ _1.23Si _0.86Al_0.82Ti_0.09)_Σ3.00O ₁₂. The largest Fe content determined in kerimasite is 21.6 wt.% Fe₂O₃ and this value corresponds to 1.66 a.p.f.u. in the tetrahedral site. Kerimasite is cubic, space group Ia3d with a = 12.549(1) Å , V = 1976.2(4) Å 3 and Z = 8. The five strongest powder-diffraction lines [d in Å , (I/Io), hkl] are: 4.441 (49) (220), 3.140 (91) (400), 2.808 (70) (420), 2.564 (93) (422) and 1.677 (100) (642). Single-crystal structure refinement revealed the typical structure of the garnet-group minerals. The name is given after the locality, Kerimasi volcano, Tanzania.