| Humboldtine | |
|---|---|
 | |
| General | |
| Category | Mineral |
| Formula (repeating unit) | FeC2O4·2H2O |
| IMA symbol | Hbd[1] |
| Crystal system | Monoclinic |
| Identification | |
| Color | Yellow to amber-yellow |
| Cleavage | Perfect on {110}, imperfect on {100} and {010} |
| Mohs scale hardness | 1.5 to 2 |
| Luster | Resinous to dull |
| Streak | Pale yellow |
| Diaphaneity | Transparent to opaque |
| Density | 2.28 g/cm3 (measured) 2.307 g/cm3 (calculated) |
| Refractive index | nα = 1.494 nβ = 1.561 nγ = 1.692 |
Humboldtine is a rarely occurring mineral from the mineral class of "organic compounds" with the chemical composition FeC2O4•2H2O and is therefore a water-containing iron(II) oxalate or the iron salt of oxalic acid.[2]
Humboldtine crystallizes in the monoclinic crystal system, but only rarely develops well-formed, tabular to prismatic crystals with a resin-like sheen on the surfaces. It is mostly found in the form of botryoidal or fibrous to earthy aggregates and crusty coatings from dull yellow to brownish yellow or amber yellow in color. It can be transparent to opaque.[3] It can form from hematite in oxalic acid.[4]
With a Mohs hardness of 1.5 to 2, humboldtine is one of the softest minerals and can be scratched with a fingernail.
- ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
- ^ Frost, R.L.; Weier, Matt L. (2004). "Thermal decomposition of humboldtine –a high resolution thermogravimetric and hot stage Raman spectroscopic study" (PDF). Archived (PDF) from the original on 2023-11-17. Retrieved 2024-01-29.
- ^ "Humboldtine". MinDat.org. Archived from the original on 20 October 2023. Retrieved 15 September 2021.
- ^ Vehmaanperä, Paula; Gong, Bo; Sit, Patrick H.-L.; Barbiellini, Bernardo; Salmimies, Riina; Häkkinen, Antti (2021). "Formation of Humboldtine During the Dissolution of Hematite in Oxalic Acid – Density Functional Theory (DFT) Calculations and Experimental Verification". Clays and Clay Minerals. 69 (6): 655–662. Bibcode:2021CCM....69..655V. doi:10.1007/s42860-021-00146-5. S2CID 238244314.