Mutinaite is the third new zeolite from Ferrar dolerites at Mt. Adamson (Northern Victoria Land, Antarctica). The mineral occurs as subspherical aggregates of tiny radiating lath-like fibers or as aggregates of transparent, colorless to pale-milky, tiny tabular crystals; it has vitreous luster, white streak and good {100} cleavage. Mutinaite is brittle with measured and calcd. d. 2.14(3) and 2.17 g/cm3, resp. Optically, it is biaxial neg. with ns α = 1.485(2), β = 1.487(2) and γ = 1.488(2). The orientation is X = b, Y = a, Z = c. Mutinaite is orthorhombic with a = 20.223(7), b = 20.052(8), c = 13.491(5)Å, space group Pnma. The strongest powder X-ray diffraction lines are (d(Å), l, hkl): 11.20, 84, 101, 011; 9.98, 35, 200,020; 3.85, 100, 501, 051; 3.75, 98, 303; 3.67, 27, 133; 3.00, 32, 503. The framework topol. is that of the synthetic zeolite ZSM-5. The chem. formula is: (Na2.76K0.11Mg0.21Ca3.78)Σ=6.86(Al11.20Si84.91)Σ=96.11O192·60H2O. The Si/Al ratio, equal to 7.6, is the highest found in a natural zeolite. Thermal stability and rehydration capacity are very high.