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R´´R´´´₂(Cb,Ta)₄O₁₄ + 4H₂O; R´´ = Fe´´, Ca; R´´´ = E; Cer = 0-2·4; Yttr = 17·2-38·3%.

Orthorhombic; isomorphous with Samarskite (q.v.).

G = 5·5-5·8. H = 5-6. Yellow to black.

Ytterby, Sweden; South Norway.