Abstract

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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.