Ogdensburgite

Chemical formula: Ca₂Fe₃+₄Zn₂+(As₅+O₄)₄(OH)₆·₆H₂O

A rare zinc and iron arsenate with a characteristic orange-red color, found in the Franklin and Sterling Hill mines, USA.

## Characteristics Ogdensburgite is a hydrated zinc, calcium, and iron arsenate. It occurs as very small, spherical aggregates or radial clusters of tiny, bladed crystals. Its most distinctive feature is its intense orange-red to yellowish-orange color. Specimens are usually small and grow on other minerals, forming thin coatings and crusts. ## Physical Properties Ogdensburgite crystals are extremely small and delicate, which makes precise determination of many physical properties difficult. The mineral exhibits a silky or dull luster. It is very soft, with a Mohs hardness of about 2. Due to its microcrystalline form, it is opaque, although individual, microscopic crystals may be translucent. ## Colors and Varieties This mineral does not have distinct varieties. Its color is uniform and ranges within a narrow spectrum from orange-red to yellowish-orange. Differences in hue primarily depend on the structure and size of the crystalline aggregates. ## History and Name Ogdensburgite was first described in 1981 by Pete J. Dunn. Its name comes from the town of Ogdensburg in New Jersey (USA), which is located near the mineral's discovery site – the famous Franklin mine. This mine, along with the neighboring Sterling Hill, constitutes the type locality for this mineral.

Properties

Mohs hardness: 2
Luster: Silky
Streak: Pale orange
Density: 3.24
Cleavage: Perfect on {010}
Fracture: Uneven
Transparency: Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Key diagnostic features of ogdensburgite include its characteristic orange-red color, occurrence as small, spherical or radial aggregates, and its extreme rarity and specific locality. The luster is usually dull or silky. Due to the size of the crystals, field identification is practically impossible and requires laboratory analysis. ## Distinguishing from Similar Minerals It can be confused with other secondary arsenates of similar color, such as pharmacosiderite or some varieties of scorodite. Differentiation requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS), due to the similarity in forms of occurrence and color. However, knowledge of associated minerals from Franklin and Sterling Hill is a crucial factor. ## Crystal Forms Ogdensburgite forms microscopic, bladed or acicular crystals that combine into spherical (spherulitic) or radial aggregates. Less frequently, it occurs as thin crusts and coatings on other minerals.

Geological environment

## Genesis It is a secondary mineral, forming in the oxidation zones of zinc deposits, in the specific, arsenic-rich environment of the metamorphic Franklin and Sterling Hill deposits. It forms as a result of the weathering of primary ore minerals, such as franklinite and willemite, in the presence of arsenic-rich solutions. ## Mineral Associations It most commonly co-occurs with minerals typical of this locality, such as willemite, franklinite, calcite, as well as other secondary arsenates, e.g., kottigite, adamite, or paradamite. It often grows directly on these minerals. ## Localities The only confirmed occurrences of ogdensburgite in the world are the Franklin mine and the Sterling Hill mine in Sussex County, New Jersey, USA. These are its type localities.

Rarity

Very rare

For collectors

## Quality Criteria The quality of an ogdensburgite specimen is primarily assessed based on the intensity and saturation of its orange-red color, and the richness and aesthetics of the aggregates on the rock matrix. Specimens with well-formed, numerous spherical aggregates that contrast with the substrate, e.g., white calcite, are more highly valued. Due to the microscopic nature of the mineral, the size of individual crystals is not a key criterion; the overall appearance and surface coverage are what matter. ## Popular Localities The only sources of specimens are the historic Franklin and Sterling Hill deposits in New Jersey, USA. All specimens available on the collector's market originate from these two, now inactive, mines.

Care and storage

## Cleaning The mineral is extremely delicate and brittle. Any mechanical cleaning, including the use of brushes, should be avoided. If absolutely necessary, the specimen can be carefully rinsed in distilled water and immediately, very gently dried with compressed air from a distance. ## What to Avoid Contact with chemicals, acids, and cleaning agents must be strictly avoided. The mineral is sensitive to changes in temperature and humidity. It should not be exposed to direct sunlight, which can cause fading and further dehydration. ## Storage Ogdensburgite specimens should be stored under stable conditions, in closed, padded boxes (e.g., "micromount" type), to protect them from mechanical damage, dust, and humidity fluctuations. Due to its fragility, it is not suitable for display outside specialized containers.

Sources

IMA Mineral List (CNMNC)