Haidingerite

Chemical formula: Ca(As⁵⁺O₃OH)·H₂O

Haidingerite is a rare, hydrated calcium arsenate, forming colorless or white, tabular crystals with perfect cleavage.

## Characteristics Haidingerite is a hydrated calcium arsenate, occurring as small, tabular or bladed crystals. It typically forms small, radial or fan-shaped aggregates, as well as coatings and crusts. Individual crystals are often transparent and colorless, with a strong vitreous or pearly luster on cleavage surfaces, which gives them a distinctive appearance. Due to its softness and perfect cleavage, it is a very brittle mineral. ## Physical Properties This mineral is characterized by a low Mohs hardness of 2-2.5, meaning it can be scratched with a fingernail. Its density is approximately 2.85 g/cm³. It exhibits perfect cleavage in one direction, causing it to easily split into thin lamellae. The luster is vitreous, and pearly on cleavage planes. It is transparent to translucent. ## Colors and Varieties Haidingerite is usually colorless or white. No colored varieties or trade names are distinguished. ## History and Name The mineral was first described in 1827 by the British mineralogist Edward Turner. The name "haidingerite" was given in honor of Wilhelm Karl Ritter von Haidinger (1795-1871), an Austrian mineralogist and geologist who made significant contributions to mineral systematics. The type locality from which the first described material originated is Jáchymov in the Czech Republic. ## Uses Haidingerite has no industrial application. It is solely a mineral of scientific and collector interest.

Properties

Mohs hardness: 2-2.5
Luster: Vitreous, Pearly
Streak: White
Density: 2.848
Cleavage: On {010}, perfect.
Fracture: Uneven
Transparency: Transparent,Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Haidingerite can be identified by its characteristic, colorless, tabular crystals, often forming radial aggregates. A key feature is its perfect cleavage in one direction, which gives it a pearly luster on parting surfaces. Low hardness (2-2.5) and a white streak are also important diagnostic clues. ## Distinguishing from Similar Minerals Haidingerite is sometimes confused with other secondary arsenates, such as pharmacolite, which is also white and soft. However, pharmacolite forms acicular or fibrous crystals and crystallizes in the monoclinic system. Gypsum can also look similar, but it has a different genesis and forms crystals of different shapes. ## Crystal Forms Haidingerite crystals are usually small, flattened, tabular, or bladed. They most often occur as radial, fan-shaped, or spherulitic aggregates. It is also found as small druses and crusts on host rocks.

Geological environment

## Genesis Haidingerite is a secondary mineral, forming in the oxidation (weathering) zones of ore deposits rich in arsenic, especially those containing arsenopyrite or other arsenides. It forms as a result of the reaction of arsenic-rich solutions with calcium minerals or carbonate rocks under conditions of low temperature and pressure. ## Mineral Associations This mineral often co-occurs with other secondary arsenates, such as pharmacolite, picropharmakolite, erythrite, annabergite, as well as with gypsum, calcite, and various iron oxides (e.g., goethite). ## Localities The most important occurrences of haidingerite include the historic type locality in Jáchymov (Czech Republic). It is also known from the Tsumeb mine in Namibia, where well-formed specimens have been found. Other confirmed localities include Bou Azzer in Morocco, the Sterling Hill mine in New Jersey (USA), and some deposits in Saxony (Germany).

Rarity

Rare

For collectors

## Quality Criteria The most prized haidingerite specimens are those with well-formed, transparent crystals forming aesthetic, radial or fan-shaped aggregates on a contrasting rock matrix. The size of the crystals, their luster, and the absence of damage are important, which is difficult to achieve due to the mineral's brittleness. Specimens from historic localities, such as Jáchymov, have additional value. ## Popular Localities Classic and most sought-after specimens come from the type locality in Jáchymov, Czech Republic. High-quality crystals have also been found in the Tsumeb mine in Namibia, which is famous for its exceptional secondary minerals. Specimens from Bou Azzer in Morocco are also valued by collectors.

Care and storage

## Cleaning Haidingerite specimens should be cleaned with the utmost care due to their brittleness and softness. The safest method is to use compressed air to remove dust. If wet cleaning is necessary, use only distilled water and a very soft brush, avoiding any pressure. After cleaning, the specimen should be immediately and thoroughly dried. ## What to Avoid Haidingerite is sensitive to acids and other chemicals. Ultrasonic cleaners, which could damage the delicate crystals, should be avoided. As a hydrated mineral, it may be sensitive to changes in temperature and humidity, which can lead to dehydration and disintegration. It should be protected from direct sunlight and high temperatures. ## Storage Haidingerite specimens should be stored in stable conditions, preferably in closed, padded boxes or display cases, to protect them from mechanical damage and dust. They should be kept away from minerals that may release moisture or react chemically.