Manganohörnesite

Chemical formula: Mn²⁺₃(As⁵⁺O₄)₂·8H₂O

A rare, hydrated manganese arsenate with a characteristic pink color, forming acicular crystals and radial aggregates.

## Characteristics Manganohörnesite is a hydrated manganese arsenate belonging to the vivianite group. It occurs as small, acicular or fibrous crystals, which often form radial, stellate, or spherical aggregates. Less commonly, it is found as small, flattened tabular crystals. Due to its delicate structure and small crystal size, it is a mineral primarily valued by collectors specializing in rare specimens. ## Physical Properties This mineral is characterized by low hardness, approximately 3 on the Mohs scale, making it very susceptible to scratching. Its luster is variable – from vitreous on crystal faces, to pearly on cleavage planes, and silky in the case of fibrous aggregates. It is transparent to translucent. The density is approximately 3.05 g/cm³. ## Colors and Varieties The dominant and most characteristic color of manganohörnesite is pink, in various shades – from pale pink to intensely pink, and even reddish-pink. No named varieties are distinguished. ## History and Name The mineral's name, given in 1957 by Paul B. Moore, refers to its chemical composition (dominance of manganese) and structural similarity to hörnesite, in which magnesium is replaced by manganese. Hörnesite, in turn, was named after Moritz Hörnes, an Austrian paleontologist. ## Uses Manganohörnesite has no industrial application. Its significance is purely scientific and collectible.

Properties

Mohs hardness
1
Color
White to colorless
Luster
Vitreous, Pearly, Silky
Streak
White
Density
2.64
Cleavage
{01`0} perfect, poor on {100}, {201}, {<mi>2</mi>01}
Fracture
Uneven
Transparency
Transparent
Crystal system
Monoclinic

Diagnostic features

## Identification Key diagnostic features include its pink color, crystal habit (small needles, radial aggregates), and occurrence environment – oxidation zones of manganese deposits. The pearly luster on cleavage surfaces is also characteristic. ## Distinguishing from Similar Minerals It can be confused with other pink arsenates or phosphates, such as erythrite or strengite. Erythrite usually has a more carmine or purplish-pink hue and is associated with cobalt deposits. Pink phosphates, like strengite, often have a different crystal habit and occur in different geological environments (e.g., in pegmatites or sedimentary rocks). ## Crystal Forms Crystals are typically elongated, acicular or fibrous, with a bladed cross-section. They form characteristic radial, stellate, spherulitic, or rosette aggregates. Less commonly, they occur as single, tabular crystals.

Geological environment

## Genesis Manganohörnesite is a secondary mineral formed in the oxidation (weathering) zones of hydrothermal ore deposits rich in manganese and arsenic. It forms as a result of the alteration of primary arsenate minerals in the presence of manganese-rich solutions. ## Mineral Associations It co-occurs with other minerals of the oxidation zone, such as sarkinite, brandtite, hausmannite, rhodochrosite, barite, and other manganese arsenates. ## Localities The most important and classic localities for this mineral are the Långban mine in Sweden and Sterling Hill in New Jersey (USA), where it was first described. It is also known from Broken Hill in New South Wales (Australia) and several locations in Japan (e.g., Gozaisho mine on Honshu Island).

Rarity

Rare

For collectors

## Quality Criteria Specimens most valued by collectors are those with well-formed, radial aggregates of intense pink color, contrasting with the dark matrix rock. The absence of damage to the delicate acicular crystals and the size of the aggregates are important. Specimens from classic localities, such as Långban or Sterling Hill, are particularly sought after. ## Popular Localities Specimens from the Swedish Långban mine and the American Sterling Hill deposit are considered classic and most desirable. Good quality material also comes from Broken Hill in Australia.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Due to the risk of damaging delicate crystals, washing in water should be avoided. If absolutely necessary, distilled water can be used, and the specimen should be immediately and very gently dried with compressed air from a distance. ## What to Avoid Avoid contact with all chemicals, including acids and detergents. The mineral is sensitive to high temperatures, which can lead to its dehydration (loss of water) and structural damage. It should be protected from prolonged exposure to strong sunlight, which can cause color fading. It is sensitive to humidity. ## Storage It is recommended to store specimens in closed, dry containers or display cases, away from heat sources and direct light. Due to its low hardness and fragility, it should be stored separately to avoid abrasion from harder minerals.

External references

Sources

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