Hemihedrite

Chemical formula: Pb²⁺₁₀Zn²⁺(Cr⁶⁺O₄)₆(SiO₄)₂(OH)₂

Hemihedrite is a very rare lead, zinc, and chromium mineral, forming microscopic, orange crystals, highly valued by collectors.

## Characteristics Hemihedrite is a complex lead and zinc chromate-silicate, belonging to the iranite group. It occurs as very small, tabular or bladed crystals, rarely exceeding 1 mm in length. It typically forms radial aggregates, rosettes, or crusts on the host rock. Its intense, orange color makes it easily noticeable despite its small size. ## Physical Properties This mineral is characterized by a relatively low hardness of 3 on the Mohs scale. It is brittle and very heavy due to its high lead content – its density is approximately 6.42 g/cm³. The crystals exhibit a vitreous to resinous luster and are transparent to translucent. ## Colors and Varieties The color of hemihedrite is its most distinctive feature. It ranges from bright orange, through reddish-brown (henna color), to almost black. The streak is a characteristic saffron-yellow color. No varieties are distinguished. ## History and Name The name "hemihedrite" refers to the hemihedral (triclinic) symmetry of its crystals. The mineral was first described in 1967 by S.A. Williams and J.W. Anthony based on samples from the Florence Mine in Pinal County, Arizona, USA, which is its type locality. ## Uses Due to its extreme rarity and microscopic crystal size, hemihedrite has no industrial applications. It is solely an object of scientific and collector interest.

Properties

Mohs hardness: 3
Luster: Vitreous
Streak: Saffron yellow (Munsell 5Y 8/10)
Density: 6.42
Transparency: Transparent,Translucent
Crystal system: Triclinic

Diagnostic features

## Identification Key diagnostic features include its bright orange color, saffron-yellow streak, high density, and characteristic small, tabular crystals forming radial aggregates. Co-occurrence with other rare lead minerals in Arizona is also a strong indicator. ## Distinguishing from Similar Minerals It can be confused with wulfenite, which can also be orange and occurs in similar environments. However, hemihedrite is much rarer, has lower hardness (3 compared to 3-3.5 for wulfenite), a different crystal habit (tabular, not square), and usually forms smaller crystals. Crocoite has a similar color but forms prismatic, acicular crystals and has a different genesis. ## Crystal Forms Crystals are very small, usually less than 1 mm. They have the form of thin tablets or blades, often with irregular outlines. Most commonly, they occur as radial aggregates, rosettes, spherical clusters, or thin crusts on the rock surface.

Geological environment

## Genesis Hemihedrite is a secondary mineral, forming in the oxidation (weathering) zones of lead and zinc ore deposits. Its formation requires a specific geochemical environment with the availability of chromate ions, which can originate from the weathering of chromite or other chromium minerals present in the surrounding rocks of the deposit. ## Mineral Associations At its type locality (Florence Mine, Arizona), it co-occurs with a number of other rare secondary minerals, such as cerussite, wulfenite, willemite, phoenicochroite, variscite, diabolite, as well as quartz and calcite. ## Localities It is an extremely rare mineral, known from only a few localities worldwide. The most important and well-known occurrences are in Arizona, USA, particularly in the Florence and Rat Tail mines in Pinal County. Outside of Arizona, its occurrence has been reported in small quantities in Chile (Veta Negra mine, Copiapó) and Iran.

Rarity

Very rare

For collectors

## Quality Criteria The most highly prized specimens are those with well-formed, sharply terminated crystals forming aesthetic, radial aggregates. The intensity and saturation of the orange color are crucial. Contrast with the rock matrix and the absence of damage to delicate crystals are also important. Due to the microscopic nature of the mineral, specimens are evaluated under magnification. ## Popular Localities By far the most sought-after specimens by collectors come from the type locality – the Florence Mine in Pinal County, Arizona, USA. These are considered the global standard for this mineral.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Due to its brittleness and rarity, wet cleaning is avoided. If absolutely necessary, distilled water can be used, followed by compressed air for drying, but from a distance and at low pressure. ## What to Avoid Avoid contact with all chemicals, especially acids, which can damage it. The mineral is sensitive to shocks and impacts. It should not be heated or subjected to ultrasonics. Prolonged exposure to strong sunlight may not be advisable, although there are no detailed studies on this topic. ## Storage Hemihedrite specimens, as microminerals, are best stored in enclosed, padded "micromount" boxes, which protect them from dust, light, and mechanical damage. Avoid storing in areas of high humidity.