Formula: (Y,Yb)2Si2O7
Origin of name: From locality; Keivy, Kola Peninsula, Russia
Crystal system: Monoclinic
Crystal class: 2/m
Twinning:
Unit cell: a = 6.845, b = 8.96, c = 4.734, β = 101.65° (Y), a = 6.84, b = 8.916, c = 4.745, β = 102.11° (Yb)
Colour: Colourless to white
Diaphaneity: Transparent
Luster: Vitreous
Habit: Crystals elongated platy and prismatic
Hardness: 4-5
Specific gravity: 5.95
Cleavage: 2; {110} perfect, {001} imperfect
Tenacity:
Streak: White
Synonyms/varieties:
Comments: Two distinct species with the extensions -(Y) and -(Yb) indicating the predominant rare earth cation in the formula; series with one another
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