Poldervaartite, Ca(Ca0.5Mn0.5)(SiO3OH)(OH), space group Pbca, a = 9.398(1), b = 9.139(2), c = 10.535(2) Å, V = 904.8(3) Å3, is a new mineral from the Wessels mine in the Kalahari manganese field of northwest Kuruman Hill, Republic of South Africa. It occurs as wheat-sheaf polycrystals to euhedral crystals associated with bultfonteinite, braunite, hausmannite, henritermierite, calcite, and hematite. Poldervaartite is colorless to milky white, has a density of 2.91(2) g/cm3 and a hardness (Mohs) of 5. Optically it is biaxial (+) with α = 1.634(2), β = 1.640(4), and γ = 1.656(2), and with pleochroic color: ηγ = bluish gray, ηβ = light gray, and ηα = colorless. The five strongest powder diffraction lines are [dI(hkl)] 3.231x(122),4.185(210), 2.8464(311), 2.3914(223), and 2.7894(131) Å.
The structure was solved by direct methods and refined to R = 0.024 using X-ray diffractometer data (1028 independent reflections). The structure is a hydroxyl nesosilicate with a single isolated Si tetrahedron and two M2+ sites. Ca is accommodated in M1, and both Ca and Mn share M2. OH groups are strongly bonded in the structure, preventing H2O loss at tempsratures below 460 °C. One OH group is bound to Si, making poldervaartite an acid silicate. The SiO3OH tetrahedron is externally regular but internally distorted. Poldervaartite is probably isostructural with dicalcium silicate α hydrate, a common phase in steam-cured Portland cement.