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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. 〔Mckie, D., Frankis, E.J., (1976) Nyerereite: A new volcanic carbonate mineral from Ol Doinyo Lengai, Tanzania. Zeitschrift fur Kristallographie 145, 73-95〕 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.〔
==Special characteristics==
Since nyerereite is very unstable when it reaches the surface it creates pseudomorphs,〔Hay, R.L., (1983) Carbonatite tuffs in the Laetolil Beds of Tanzania and the Kaiserstuhl in Germany. Contributions to Mineralogy and Petrology 82, 403-406〕 which is basically the process by which the rock appearance and dimensions remain constant but the main mineral component is replaced by another.〔 McKie (1976) categorized nyerereite into two different categories, high and low nyerereite. There are two types because the Ol Doinyo Lengai lavas are very soluble and hygroscopic; when they come in contact with water or the atmosphere, the lava changes physically and chemically. Therefore, when nyerereite is at high temperature or warm we have what McKie (1976) calls high nyerereite, but when it cools down and gets hydrated we have pirssonite that has a chemical formula of Na2Ca(CO3)2·2(H2O).〔
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