Olmiite, ideally CaMn[SiO3(OH)](OH), is a newly identified mineral from the N’Chwaning II mine of the Kalahari manganese fields (Republic of South Africa), which occurs as a product of hydrothermal alteration associated with poldervaartite, celestine, sturmanite, bultfonteinite and hematite. The mineral occurs as wheat-sheaf aggregates consisting of pale to intense reddish pink minute crystals. Olmiite is transparent with vitreous lustre, and exhibits deep-red fluorescence under short-wave UV-light. The mineral is brittle, with irregular fracture. Streak is white and Mohs hardness is 5–53. No cleavage was observed. The measured density (pycnometer method) is 3.05(3) g/cm3. The calculated density is 3.102 g/cm3 or 3.109 g/cm3 using the unit-cell volume from single-crystal or powder data, respectively. Olmiite is biaxial positive, with refractive indices α = 1.663(1), β = 1.672(1), γ = 1.694(1) (589 nm), 2Vmeas = 71.8(1)°, 2Vcalc = 66(8)°. The optical orientation is X = a, Y = c, Z = b and dispersion: r > v, distinct. Pleochroism is not observed. Chemical analysis by electron microprobe yielded the chemical formula (Ca2–xMnxFey)[SiO3(OH)](OH), with 0.84 ⩽ x ⩽ 0.86, and y ⩽ 0.01. Olmiite is orthorhombic, space group Pbca, with a = 9.249(3), b = 9.076(9), c = 10.342(9) Å, V = 868(1) Å3 and Z = 8. The strongest five powder-diffraction lines [d in Å, (I/Io), hkl] are: 4.14, (45), 021; 3.19, (100), 122; 2.807, (35), 311; 2.545 (35), 312; 2.361, (40), 223. Single-crystal structure refinement (R1 = 2.74% for 1012 observed reflections) showed that the atomic arrangement of olmiite is similar to that of poldervaartite, with all Mn ordered on the M2 site. Significant variations in bond distances and angles are related to the pronounced difference in the Mn content. Olmiite, therefore, is the Mn-dominant analogue of poldervaartite. The name poldervaartite should be reserved for samples having Ca dominant at the M2 site.