Hörnesite

Chemical formula: Mg₃(As⁵⁺O₄)₂·₈H₂O

Hörnesite is a hydrated magnesium arsenate of the vivianite group, forming white, acicular crystals with a silky luster.

## Characteristics Hörnesite is a hydrated magnesium arsenate belonging to the vivianite group. It most often occurs as acicular, fibrous, or radial aggregates. Well-formed crystals are rare, usually taking the form of small, flattened or bladed needles. The mineral is typically white and opaque in mass, but individual crystals can be colorless and transparent. ## Physical Properties Hörnesite is a very soft mineral, with a Mohs hardness of just 1, meaning it can be scratched with a fingernail. It has perfect cleavage in one direction, causing it to easily break into thin lamellae or fibers. It is characterized by a silky or pearly luster, especially on cleavage surfaces. Its density is approximately 2.57 g/cm³. ## Colors and Varieties This mineral is typically white. In transmitted light, individual crystals or thin fragments become colorless. No colored varieties or trade names are distinguished. ## History and Name The name hörnesite comes from the surname of Moritz Hörnes (1815-1868), an Austrian paleontologist and curator of the Imperial Mineralogical Cabinet in Vienna. The mineral was first described in 1845 based on specimens found in the historic mine in Ciclova (Csiklova), Romania, which is its type locality. ## Uses Due to its rarity, small crystal size, and low hardness, hörnesite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals or arsenate minerals.

Properties

Mohs hardness: 1
Luster: Pearly, Silky
Streak: White to colorless
Density: 2.57
Cleavage: On {010}, perfect; on {100}, poor.
Fracture: Uneven
Transparency: Transparent
Crystal system: Monoclinic

Diagnostic features

## Identification Hörnesite can be identified by its characteristic white, acicular or fibrous aggregates. Key features include very low hardness (1 on the Mohs scale), silky or pearly luster, and perfect cleavage in one direction. The streak is white. ## Distinguishing from Similar Minerals - **Annabergite:** This is a nickel arsenate that forms very similar crystals but has a characteristic light green color. - **Erythrite:** A cobalt arsenate, also from the vivianite group, is similar in form but has an intense, carmine or reddish-pink color. - **Gypsum (especially selenite):** Can form similar colorless, bladed crystals, but has a higher hardness (2) and different genesis. ## Crystal Forms Hörnesite crystallizes in the monoclinic system. It forms small, acicular or bladed crystals, often flattened. It most often occurs as radial or fibrous aggregates, as well as crusts.

Geological environment

## Genesis Hörnesite is a rare secondary mineral, formed in the oxidation (weathering) zones of ore deposits containing arsenic, in magnesium-rich rocks such as dolomites or serpentinites. It forms as a result of the reaction of solutions rich in arsenic acid with magnesium minerals. ## Mineral Associations It often co-occurs with other arsenates, such as annabergite and erythrite, as well as with ore minerals (arsenopyrite, smaltite), carbonates (dolomite, calcite), and sulfates (gypsum). ## Localities The most important occurrences of hörnesite worldwide include: - **Romania:** Ciclova Română (historic type locality). - **Germany:** In the Schneeberg and Johanngeorgenstadt regions of Saxony. - **Greece:** Mines in the Laurion (Lavrio) region. - **USA:** Sterling Hill in New Jersey. - **Japan:** Kiura Mine in Oita Prefecture.

Rarity

Rare

For collectors

## Quality Criteria For collectors, the most valuable specimens are those with well-formed, distinct crystals, which is rare for this mineral. Rich, radial aggregates with a pure white color and silky luster, contrasting with the rock matrix, are highly prized. The size of the crystals and the overall specimen also significantly influence its attractiveness. ## Popular Localities The most classic and sought-after specimens by collectors come from historical localities in Europe, such as Laurion in Greece and Schneeberg in Germany. Specimens from these locations are valued for their historical significance and often good crystal quality.

Care and storage

## Cleaning Hörnesite specimens are extremely delicate and must be handled with the utmost care. Cleaning should be kept to an absolute minimum. If necessary, a very soft brush can be used to remove dust. Avoid contact with water, which can damage or dissolve the mineral. ## What to Avoid - **Water and Moisture:** Hörnesite is a hydrated arsenate and can react to changes in humidity. Avoid washing in water. - **Shocks and Impacts:** Due to its extremely low hardness (1 on the Mohs scale) and perfect cleavage, the mineral is very brittle and susceptible to mechanical damage. - **Chemicals:** Avoid contact with acids, bases, and other chemical agents. ## Storage Hörnesite specimens should be stored in stable conditions, in a dry place, away from direct sunlight and heat sources. It is best to place them in a sealed box with soft padding to protect them from dust and mechanical damage. Do not store it with harder minerals that could scratch it.