Simpsonite
Chemical compound
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Simpsonite has a general formula of Al₄(Ta, Nb)₃O₁₃. It occurs as euhedral to subhedral tabular to short and prismatic crystals, commonly in subparallel groups. Under the petrographic microscope it has a very high relief. Wikipedia
Crystal class: Pyramidal (3); H-M symbol: (3)
Crystal habit: Euhedral, prismatic, striated
Crystal system: Trigonal
Space group: P3