Hydroniumpharmacosiderite

Chemical formula: (H<sub>3</sub>O)Fe<sup>3+</sup><sub>4</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>3</sub>(OH)<sub>4</sub>·4H<sub>2</sub>O

Hydroniumpharmacosiderite is a rare iron arsenate, distinguished by its regular, cubic crystals with variable color from yellow to red.

## Characteristics Hydroniumpharmacosiderite is a hydrated iron arsenate belonging to the pharmacosiderite group. It forms characteristic, well-developed crystals with an isometric, most often cubic habit. Crystals are usually small but can form visually attractive, lustrous aggregates and coatings on the host rock. Its appearance is very similar to its related mineral, pharmacosiderite. ## Physical Properties Crystals exhibit a hardness of approximately 2.5 on the Mohs scale, making them a soft mineral. They have a luster ranging from vitreous to slightly resinous. They are usually translucent, and their density is about 2.75 g/cm³. ## Colors and Varieties This mineral occurs in various shades, most commonly from yellow, through yellowish-green, greenish-brown, to reddish-brown. The color depends on the degree of hydration and potential impurities. No distinct commercial or gemological varieties are recognized. ## History and Name The mineral's name, approved in 2012, refers to its chemical composition – the presence of hydronium ions (H₃O⁺) – and its structural and chemical similarity to pharmacosiderite. It is a relatively newly defined member of the pharmacosiderite group, distinguished during a revision of this mineral group. ## Uses Hydroniumpharmacosiderite has no industrial application. Its significance is purely scientific and collectible, as a rare and interesting representative of the arsenate group.

Properties

Mohs hardness
2.5
Luster
Vitreous to Resinous
Streak
Pale yellow
Density
2.75
Cleavage
Imperfect on {001}
Fracture
Uneven
Transparency
Translucent
Crystal system
Cubic

Diagnostic features

## Identification The key diagnostic feature is the characteristic, regular (most often cubic) crystals with colors ranging from yellow to brown, found in the oxidation zones of ore deposits. Vitreous to resinous luster and low hardness are also helpful in identification. ## Distinguishing from Similar Minerals Distinguishing hydroniumpharmacosiderite from other minerals of the pharmacosiderite group (e.g., potassium pharmacosiderite) is visually impossible and requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS/WDS). It can also be confused with other secondary arsenates, such as scorodite; however, scorodite forms crystals of a different shape (usually dipyramidal or tabular) and crystallizes in the orthorhombic system. ## Crystal Forms This mineral crystallizes in the isometric system, forming almost exclusively cubic crystals. These crystals occur as single, scattered on the rock matrix, or form dense aggregates and coatings.

Geological environment

## Genesis Hydroniumpharmacosiderite is a secondary mineral, forming in the oxidation (weathering) zones of ore deposits rich in arsenic, especially arsenopyrite and other iron sulfoarsenides. It forms under low temperature and pressure conditions, as a result of surface waters acting on primary ore minerals. ## Mineral Associations It most often co-occurs with other secondary arsenate minerals, such as scorodite, pharmacosiderite, beudantite, as well as with iron oxides and hydroxides, like goethite and hematite. It may also accompany relics of primary sulfide minerals, e.g., pyrite and arsenopyrite. ## Localities Confirmed occurrences of this mineral are few. The most important include historical mines in Cornwall (Great Britain), Utah (USA), as well as in Greece (mines in the Laurion region) and Germany (Black Forest). These are classic localities for many rare, secondary arsenate minerals.

Rarity

Rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with sharp, well-formed, lustrous crystals of intense color (e.g., vivid yellow or reddish-brown). Contrast with the rock matrix and the size and richness of crystal aggregates are important. Specimens from classic, historical localities, such as Cornwall, are particularly sought after. ## Popular Localities Among the best-known and most valued localities from which hydroniumpharmacosiderite specimens originate are the mines in the St Day and Gwennap area in Cornwall (Great Britain) and deposits around Laurion in Greece. Specimens from these places are considered classics of the species.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using only a soft brush to remove dust. Due to its low hardness and potential reactivity, contact with water, and especially with acids and detergents, should be avoided. ## What to Avoid Avoid ultrasonic cleaners, chemical agents, and sudden temperature changes. The mineral is sensitive to heat and can undergo dehydration (loss of water), leading to changes in color and structure. It should be protected from direct sunlight. ## Storage It is recommended to store specimens in stable conditions, in a dry place, away from heat and light sources. It is best to keep them in closed collector's boxes to protect delicate crystals from mechanical damage and dust.

Sources

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