Humboldtine

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
General
Category	Minerals
Formula	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 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.

Etymology and history

Humboldtine was first discovered by August Breithaupt in a weathered brown coal deposit near the municipality of Korozluky in Okres Most in the Czech Republic and described in 1821 by Mariano Eduardo de Rivero y Ustariz (1798-1857),^[5] who named the mineral after the German naturalist Alexander von Humboldt.

The mineral was already known and characterized when the IMA was founded (1959). Accordingly, humboldtine is listed as an officially recognized mineral.

Occurrence

The mineral is known to be found in 30 localities, including Germany, Brazil, the U.K. and Canada.

In 2023, during the digitization of the archive of the Bavarian Environment Agency (LfU) [de], a 75 year old letter from a coal mine owner written in 1949 was found. It mentioned that mineral was found in coal mine near the town of Schwandorf. Analysis of the mineral was done which confirmed the discovery. The discovery doubled the amount of Humboldtine known so far.^[6]

Crystal structure

Humboldtine crystallizes monoclinically in the space group C2/c (space group no. 15) with the lattice parameters a = 12.011 Å; b = 5.557 Å; c = 9.920 Å and β = 128.53°, with four formula units per unit cell.

Uses

It is an important synthetic intermediate and also a key building block for the preparation of various advanced materials.^[7]