Humboldtine

Chemical formula: Fe²⁺(C₂)⁶⁺O₄·₂H₂O

Humboldtine is a hydrated iron(II) oxalate, forming characteristic, silky aggregates of an intensely yellow color.

## Characteristics Humboldtine is a mineral from the organic group, specifically a hydrated iron(II) oxalate. It rarely forms well-developed, small crystals with a prismatic or acicular habit. Most often, it occurs as fibrous, radial, earthy aggregates, or as crusts and coatings. Its appearance is very characteristic – it forms delicate, silky aggregates resembling felt, often with an intense, lemon-yellow to honey-yellow color. ## Physical Properties This mineral is very soft; its Mohs hardness is 1.5-2, meaning it can be scratched with a fingernail. It is also very light, with a density of approximately 2.2 g/cm³. It exhibits a silky luster on fibrous aggregates and a vitreous luster on crystal faces. It is translucent to opaque. ## Colors and Varieties The dominant and virtually sole color of humboldtine is intensely yellow, in shades ranging from lemon, through golden-yellow, to honey and yellowish-brown. No color or commercial varieties are distinguished. ## History and Name The mineral's name, given in 1821 by Mariano de Rivero, honors the famous German naturalist and explorer, Alexander von Humboldt (1769-1859). The discovery was made in Colombia, near the town of Tolima. ## Uses Due to its rarity, small size, and low hardness, humboldtine has no industrial applications. It is solely an object of interest for collectors of rare and unusual minerals.

Properties

Mohs hardness: 1.5-2
Color: Yellow to amber-yellow; lemon-yellow
Luster: Vitreous, Silky
Streak: Pale yellow
Density: 2.28
Cleavage: On {110},perfect; on {100} and {010}, imperfect.
Fracture: Uneven
Transparency: Transparent,Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification The key diagnostic feature of humboldtine is its intense, lemon-yellow color combined with very low hardness (it can be scratched with a fingernail) and its characteristic occurrence in the form of delicate, silky or felt-like aggregates. ## Distinguishing from Similar Minerals It can be confused with other secondary sulfate or uranium minerals of yellow color. It is distinguished from sulfates (e.g., jarosite) by its extremely low hardness. It is distinguished from uranium minerals (e.g., autunite, carnotite) by its lack of radioactivity and different genesis. The reaction with acids is also different. ## Crystal Forms Crystals, if present, are very small, prismatic or acicular. It usually forms fibrous, radial, spherical aggregates, as well as earthy masses, coatings, and thin crusts.

Geological environment

## Genesis Humboldtine is a secondary mineral. It forms as a result of the interaction of solutions rich in oxalic acid (derived from the decomposition of organic matter, e.g., lignite, peat) with iron minerals, such as pyrite or siderite. It is a typical mineral of the oxidation zones of ore deposits or coal seams. ## Mineral Associations It co-occurs with minerals such as melanterite, gypsum, epsomite, siderite, pyrite, as well as with lignite and peat. ## Localities Important worldwide localities include those in the Czech Republic (near Most and Sokolov), Germany (Hesse, Saxony), Hungary (Pécs-Vasas mine), Italy (Tuscany), Canada (Yukon), and the USA (North Dakota).

Rarity

Not very common

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, albeit small, crystals. Rich, intensely colored, silky fibrous aggregates covering a significant area of the host rock are also highly valued. Contrast with a dark matrix (e.g., coal) enhances the visual appeal of the specimen. ## Popular Localities Classic and valued specimens come from historical localities in the Czech Republic (e.g., the Bílina lignite mine) and Germany. Specimens from these locations are considered exemplary for this species.

Care and storage

## Cleaning Humboldtine specimens are extremely delicate and sensitive. They should be cleaned only dry, using a very soft brush to remove dust. Any contact with water, and especially with chemicals, is highly inadvisable. ## What to Avoid This mineral is sensitive to moisture and can dehydrate in overly dry environments. Direct sunlight, which can cause fading, should be avoided. It is extremely sensitive to all acids and bases. Due to its low hardness, it must be protected from any physical contact and vibrations. ## Storage It is recommended to store specimens in closed, airtight containers (e.g., "membrane box" type or in boxes with cotton) to protect them from mechanical damage, dust, and humidity fluctuations. Exposure should be away from light and heat sources.