Arsenohopeite
Chemical formula: Zn<sup>2+</sup><sub>3</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>·4H<sub>2</sub>O
Arsenohopeite is a rare, hydrated zinc arsenate, forming colorless or white crystals, and is the arsenate analog of hopeite.
Description
## Characteristics Arsenohopeite is the arsenate analog of hopeite, belonging to the phosphate and arsenate group. It forms small, well-formed, prismatic crystals, often occurring in radial or divergent aggregates and crusts (druses). It is typically colorless to white, and its luster and transparency make it an attractive micromount mineral. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of about 3. It exhibits a vitreous luster, which transitions to pearly on excellent cleavage surfaces. It is transparent to translucent. Its density is approximately 3.86 g/cm³, which is a relatively high value resulting from the presence of zinc and arsenic. ## Colors and Varieties Arsenohopeite occurs in a limited color range, from completely colorless to milky white. No named color varieties or commercial variants are distinguished. ## History and Name The mineral was first described in 1965 by L. G. Berry and B. D. M. Mason. Its name directly refers to its chemical composition – the presence of arsenic ("arseno") – and its structural and chemical similarity to hopeite ("hopeite"). ## Uses Arsenohopeite has no industrial applications. It is solely an object of scientific interest and a valued acquisition in advanced systematic mineral and micromount collections.
Diagnostic features
## Identification Helpful features for identifying arsenohopeite include its crystal forms (short prisms, often in radial aggregates), colorless or white color, vitreous-pearly luster, and perfect cleavage in one direction. The environment of occurrence – oxidation zones of zinc-bearing deposits – is also crucial. ## Distinguishing from Similar Minerals Arsenohopeite is visually almost indistinguishable from hopeite (its phosphate analog) – this requires chemical analysis (EDS) to confirm the dominance of arsenic over phosphorus. It can be confused with other colorless or white secondary minerals, such as parascholzite (crystallizes in the monoclinic system) or hemimorphite (usually forms different crystal habits and has higher hardness). ## Crystal Forms Arsenohopeite crystals are usually small, with a short-prismatic habit. It most often forms radial aggregates, rosettes, and microcrystalline crusts (druses) on the surface of the host rock or other minerals.
Geological environment
## Genesis Arsenohopeite is a secondary mineral, forming in the oxidation (weathering) zones of hydrothermal ore deposits that are rich in both zinc and arsenic. It crystallizes in rock cavities and fissures as a result of surface water acting on primary ore minerals. ## Mineral Associations This mineral often co-occurs with other secondary zinc arsenates and phosphates. Its typical associations include: spencerite, parascholzite, scholzite, tarbuttite, as well as hemimorphite, hydrozincite, and smithsonite. ## Localities The type locality is the Hudson Bay Mine in Salmo (British Columbia, Canada). However, the most famous and prized specimens come from two world-class secondary mineral localities: the Tsumeb Mine in Namibia and the Kabwe Mine (formerly Broken Hill) in Zambia.
Rarity
Very rare
Collector aspects
## Quality Criteria The most highly valued arsenohopeite specimens are characterized by sharply terminated, transparent crystals with a distinct luster, grouped into aesthetic, radial aggregates. The absence of damage and a contrasting matrix are important. Crystal size rarely exceeds a few millimeters, so even small but well-formed specimens are valuable. ## Popular Localities Specimens from the Tsumeb Mine in Namibia and Kabwe in Zambia are considered the best in the world. They are characterized by exceptional crystal quality and often occur in association with other rare minerals, which further increases their collector's value.
Care and storage
## Cleaning Arsenohopeite specimens should be cleaned very carefully, using a soft brush and distilled water. Ultrasonic cleaners should be avoided, as they can damage delicate crystals and cause fractures along cleavage planes. ## What to Avoid The mineral is sensitive to shocks and impacts due to its low hardness and perfect cleavage. Contact with acids, which can damage it, should be avoided. As an arsenate, it is toxic – hands should be washed after any contact with the specimen, and inhalation of mineral dust should be avoided. ## Storage It is recommended to store specimens in separate, sealed boxes (such as "membrane boxes" or padded containers) to protect them from dust, mechanical damage, and contact with other minerals.