Hentschelite

Chemical formula: Cu²⁺Fe³⁺₂(PO₄)₂(OH)₂

Hentschelite is a rare copper and iron phosphate, forming characteristic, almost black crystals with a strong, almost metallic luster.

## Characteristics Hentschelite is a mineral from the phosphate group, chemically a hydrated copper and iron phosphate. It occurs in the form of very small, but well-formed crystals, often with a pseudo-octahedral shape. Its color is so dark – from greenish-black to black – that individual crystals are difficult to observe without magnification. It most often forms coatings and crystalline druses in rock fissures. ## Physical Properties Hentschelite crystals, despite their small size, are characterized by high hardness, measuring 3.5 on the Mohs scale. They have a strong, vitreous to almost metallic luster. They are opaque or only translucent on the thinnest edges. The mineral's density is relatively high, approximately 3.95 g/cm³. ## Colors and Varieties This mineral does not exhibit color variation. Its color is invariably dark, from black to very dark green. No color or commercial varieties are distinguished. ## History and Name The name hentschelite was given in honor of Gerhard Hentschel (1930-1997), a German amateur mineralogist and mineralogical curator, who made significant contributions to the study of the mineralogy of the Eifel region in Germany. The mineral was first described in 1984 based on samples from the Reiche Zeche mine near Freiberg in Saxony (Germany). ## Applications Due to its rarity and occurrence in the form of small crystals, hentschelite has no industrial application. It is solely an object of interest for collectors specializing in rare minerals or micromounts.

Properties

Mohs hardness
3.5
Color
Dark green to greenish black
Luster
Vitreous to sub-metallic
Streak
Pale green
Density
0
Cleavage
Indistinct on {010}
Fracture
Irregular/Uneven
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Characteristic features of hentschelite include its black or very dark green color, strong luster, and occurrence in the form of small, shiny crystals with a pseudo-octahedral shape. The environment of occurrence is also crucial – oxidation zones of ore deposits, often in association with other secondary phosphates. ## Distinguishing from Similar Minerals It can be confused with other dark secondary minerals, such as tenorite (CuO) or some varieties of goethite. However, hentschelite is distinguished by its specific crystal shape and a greenish tint visible under strong illumination. Final differentiation from lazulite, with which it forms a series, requires advanced chemical (EDS) or X-ray (XRD) analysis. ## Crystal Forms Crystals are usually very small, rarely exceeding 1 mm. They take pseudo-octahedral or wedge-shaped forms. Most often, they form granular aggregates, coatings, or druses of small, shiny crystals in fissures and on the surface of other minerals.

Geological environment

## Genesis Hentschelite is a secondary mineral, forming in the oxidation (weathering) zones of ore deposits, mainly copper. It forms as a result of the alteration of primary minerals containing copper, iron, and phosphorus in an oxygen-rich environment. ## Mineral Associations It often co-occurs with other secondary phosphate and sulfate minerals. Its typical associated minerals include: rockbridgeite, strengite, chalcosiderite, thuringite, quartz, barite, as well as minerals from the lazulite-scorzalite group, with which it forms a solid solution. ## Localities The most important and classic localities for this mineral are in Germany, from where the type specimen originates (Reiche Zeche mine, Freiberg, Saxony) and from the Rothläufchen quarry in Waldgirmes (Hesse). It is also known from Portugal (Miguel Vacas mine), Spain (El Tesorero), as well as from rare occurrences in Australia and the United States (e.g., New Hampshire).

Rarity

Rare

For collectors

## Quality Criteria The most highly prized hentschelite specimens are those that feature rich druses with numerous, well-formed, and shiny crystals, clearly contrasting with the host rock (matrix). Color contrast, for example with white quartz or barite, significantly enhances visual appeal. Crystal size, though always small, is an important factor – specimens with crystals approaching 1 mm are considered exceptional. ## Popular Localities The best specimens in the world are considered to come from the German locality of Waldgirmes (Rothläufchen quarry). They are characterized by exceptionally sharp and shiny crystals, often on a contrasting substrate. Specimens from the type locality (Freiberg) are primarily of historical significance.

Care and storage

## Cleaning Hentschelite specimens should be cleaned very carefully, preferably using compressed air to remove dust. If necessary, a soft brush and distilled water can be used, followed by thorough drying of the specimen. Ultrasonic cleaners should be avoided, as they can damage delicate crystals. ## What to Avoid The mineral is sensitive to strong acids, which can damage it. Contact with household and laboratory chemicals should be avoided. It should not be heated or exposed to sudden temperature changes. ## Storage Collector's specimens of hentschelite, especially those in the form of micromounts, are best stored in sealed, transparent boxes (e.g., "micromount boxes") that protect them from dust, moisture, and mechanical damage. Exposure to direct sunlight is not recommended, although the mineral is not particularly light-sensitive.

External references

Sources

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