Ungemachite

Chemical formula: K<sub>3</sub>Na<sub>8</sub>Fe<sup>3+</sup>(S<sup>6+</sup>O<sub>4</sub>)<sub>6</sub>(N<sup>5+</sup>O<sub>3</sub>)<sub>2</sub>·6H<sub>2</sub>O

A very rare, water-soluble sulfate and nitrate of sodium, potassium, and iron, forming small, tabular, brown crystals.

## Characteristics Ungemachite is a complex hydrated sulfate and nitrate of sodium, potassium, and iron. It forms small, most often tabular or rhombohedral crystals, which rarely exceed a few millimeters. They usually occur as single, well-formed crystals or in small aggregates growing on other minerals. Its most characteristic feature is its high solubility in water. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 2.5, meaning it can be scratched with a fingernail. It has a density of 2.72 g/cm³, typical for salt minerals. It exhibits a vitreous luster and is transparent to translucent. It is brittle and shows perfect cleavage in one plane. ## Colors and Varieties Ungemachite ranges in color from yellowish-brown, through reddish-brown, to brown. Its color is quite uniform, and it does not form colored varieties or possess special trade names. ## History and Name The mineral was described in 1936 by Martin A. Peacock and Mark Chance Bandy. They named it in honor of Henri-Léon Ungemach (1880–1936), a French crystallographer and mineralogist from Strasbourg, who was the first to study in detail crystals from the Chilean nitrate deposits where ungemachite was discovered. ## Uses Due to its rarity, small crystal size, and sensitivity to moisture, ungemachite has no industrial applications. It is solely an object of scientific interest and a prized acquisition in advanced systematic mineral collections.

Properties

Mohs hardness
2.5
Luster
Vitreous
Streak
Pale yellow
Density
2.72
Cleavage
Perfect on {0001}
Fracture
Conchoidal
Transparency
Transparent to Translucent
Crystal system
Trigonal

Diagnostic features

## Identification The key diagnostic feature of ungemachite is its immediate solubility in a drop of water (a destructive test, to be performed on a micro-fragment). In field conditions and in a collection, it is recognized by its small, tabular or rhombohedral brown crystals with a vitreous luster. Its perfect basal cleavage is also characteristic. ## Differentiation from Similar Minerals It can be confused with other rare sulfates and nitrates from Chilean saltpeter deposits, such as humberstonite or bloedite. Differentiation requires precise analysis of crystal shape (trigonal), hardness (it is very soft), and specific mineral associations. Definitive identification often requires chemical analysis. ## Crystal Forms Crystals are usually small, with a tabular habit (flattened parallel to the base) or rhombohedral. They occur as single, overgrown crystals or in small, granular aggregates.

Geological environment

## Genesis Ungemachite is a secondary mineral, formed in extremely arid climatic conditions. It forms as a product of evaporation of nitrate and sulfate-rich solutions in Chilean saltpeter deposits. It occurs as an efflorescence on the surface of other salt minerals. ## Mineral Associations It co-occurs with other minerals characteristic of nitrate deposits, such as nitratine (sodium nitrate), humberstonite, bloedite, glauberite, and halite. ## Localities The most important and classic locality for ungemachite is the nitrate deposits in the Atacama Desert in Chile, particularly in the Antofagasta region (e.g., Oficina Paposo, Taltal). This is practically the only source in the world for well-formed specimens of this mineral.

Rarity

Very rare

For collectors

## Quality Criteria The most highly valued specimens are those with sharp, well-formed, transparent crystals of intense, brown color. Due to the generally small size of the crystals, size is a key factor influencing value – specimens with crystals exceeding 2-3 mm are already considered exceptional. Attractiveness is also enhanced by a contrasting placement on a matrix composed of other salt minerals, e.g., white halite. ## Popular Localities Practically all collector specimens come from the historical nitrate fields (so-called "oficinas") in the Antofagasta region of northern Chile. Localities such as Paposo or Sierra Gorda have provided the best known specimens.

Care and storage

## Cleaning Ungemachite is extremely sensitive to water and dissolves very easily in it. **Under no circumstances should it be wet cleaned.** To remove dust, use only a soft, dry brush or a very gentle stream of compressed air. ## What to Avoid Contact with water, water vapor, and high air humidity must be strictly avoided, as these can cause the specimen to dissolve and be destroyed. The mineral is hygroscopic, meaning it actively absorbs moisture from the environment. It should also be protected from acids and sudden temperature changes. ## Storage Ungemachite specimens must be stored in conditions of controlled, low humidity. The best solution is a tightly sealed container (e.g., a "perky box") with a desiccant inside, such as silica gel. Avoid exposure to open air.

External references

Sources

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