Vaterite
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Vaterite is a mineral, a polymorph of calcium carbonate. It was named after the German mineralogist Heinrich Vater. It is also known as mu-calcium carbonate. Vaterite belongs to the hexagonal crystal system, whereas calcite is trigonal and... Wikipedia
Birefringence: δ = 0.100
Color: Colorless
Crystal class: Dihexagonal dipyramidal (6mmm); H-M symbol: (6/m 2/m 2/m)
Crystal habit: Fine fibrous crystals, typically less than 0.1 mm, in spherulitic aggregates
Crystal system: Hexagonal
Formula (repeating unit): CaCO3
IMA symbol: Vtr