Formula: (Ce,Y,Ca)5(SiO4,PO4)3(OH,F)
Origin of name: From Greek for weight
Crystal system: Hexagonal
Crystal class: 6/m
Twinning:
Unit cell: -Ce: a = 9.58 Å, b = 9.59 Å, c = 6.98 Å, b = 89.7°, -Y: a = 9.422 Å, b = 9.414 Å, c = 9.764 Å
Colour: Brown, yellow-brown, blue
Diaphaneity: Transparent in thin flakes, opaque
Luster: Adamantine
Habit: Crystals, hexagonal prisms to 1 cm; massive, granular
Hardness: 5
Specific gravity: 4.2 to 4.69
Cleavage: Imperfect on {0001} and {10-10}
Tenacity: Brittle, conchoidal fracture (-Ce), uneven to splintery fracture (-Y)
Streak: Pale brown
Synonyms/varieties: Pravdite, Lessingite-(Ce), Beckelite, Abukumalite
Comments: Two distinct species with the extensions -(Ce) and -(Y) indicating the predominant rare earth cation in the formula. Mainly found in pegmatites and contact deposits related to them
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