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(mineralogy) A rare monoclinic-prismatic mineral containing carbon, hydrogen, iron and oxygen.

Humboldtine – Minerals – FeC₂O₄·2H₂O – Hbd – Monoclinic – Yellow to amber-yellow – Perfect on {110}, imperfect on {100} and {010} – 1.5 to 2 – Resinous to dull – Pale yellow – Transparent to opaque – 2.28 g/cm³ (measured) – 2.307 g/cm³ (calculated) – Nα = 1.494 – Nβ = 1.561 – Nγ = 1.692.