Nyerereite

Nyerereite is a very rare sodium calcium carbonate mineral with formula Na2Ca(CO3)2. It forms colorless, platey pseudohexagonal orthorhombic crystals that are typically twinned. It has a specific gravity of 2.54 and indices of refraction of nα=1.511, nβ=1.533 and nγ=1.535. Nyerereite is not stable in contact with the atmosphere and rapidly breaks down. Collection specimens must be kept in a sealed argon environment. It has a Hermann–Mauguin notation of mm2 and the respective space group is Cmc21. In nature Nyerereite is naturally twinned and is pseudohexagonal with triad twinning; meaning that this is a six sided crystal that apparently has a hexagonal shape but is not in the hexagonal system. Triad twinning is the intergrowth of three orthorhombic crystals that turn at their center and form hexagonally shaped crystals. Nyerereite is biaxial negative, and has a 2v of 29 degrees. It shows a center acute bisectrix and a birefringence of approximately 0.023. At high temperatures or just erupted lava nyerereite is uniaxial and shows an interference color of second order blue when twinning is not present, and when twinning is there the interference color of nyerereite is first order grey.