Segerstromite
Chemical formula: Ca₃(As⁵⁺O₄)₂(As³⁺(OH)₃)₂
Segerstromite is an extremely rare calcium arsenate, forming colorless to pale yellow, tabular crystals with a vitreous luster.
Properties
- Mohs hardness
- 4.5
- Color
- Colorless
- Luster
- Vitreous
- Streak
- White
- Density
- 3.46
- Cleavage
- Perfect on {010}
- Fracture
- None observed
- Transparency
- Transparent
- Crystal system
- Isometric
Diagnostic features
## Identification Segerstromite is difficult to identify visually without specialized equipment. It is recognized by its characteristic form of small, colorless to pale yellow, tabular crystals with a vitreous luster, occurring in a specific geological environment. Final identification requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS/WDS). ## Distinguishing from similar minerals It can be confused with other colorless or pale yellow secondary minerals found in oxidation zones, such as pharmacolite or gypsum. It is distinguished from them by its higher density and specific crystallography (monoclinic system). Its occurrence in association with other arsenates in Franklin is a key indicator. ## Crystal forms Crystals are typically tabular, flattened parallel to {010} and elongated along [100]. They form thin, bladed shapes, often in small aggregates or as single, isolated crystals on the surface of the host rock.
Geological environment
## Genesis Segerstromite is a secondary mineral, formed by hydrothermal processes in altered (metamorphosed) zinc, iron, and manganese deposits. At the Sterling Mine, it formed in fractures of calcite veins cutting through franklinite-willemite ore. ## Mineral associations This mineral co-occurs with minerals such as: franklinite, willemite, zincite, calcite, magnetite, hematite, as well as other rare arsenates, e.g., chlorophoenicite. ## Localities The only confirmed locality for segerstromite in the world is its type locality: Sterling Mine, Sterling Hill, Ogdensburg, Sussex County, New Jersey, USA. This is one of the most famous mineralogical sites in the world, renowned for its unique zinc and manganese mineralization and the occurrence of many rare mineral species.
Rarity
Extremely rare
For collectors
## Quality criteria The quality of a segerstromite specimen is primarily assessed based on the presence of well-formed, sharp, and undamaged crystals. Due to their small size, even millimeter-sized crystals are considered valuable. Specimens where crystals are clearly isolated and contrast with the rock matrix are more highly prized. The presence of associated minerals can enhance the specimen's attractiveness. ## Popular localities The only source of segerstromite specimens is the Sterling Mine in Ogdensburg, New Jersey, USA. All specimens available on the collector's market originate from this single, historic locality.
Care and storage
## Cleaning Due to its extreme rarity and delicacy, mechanical cleaning is highly inadvisable. If absolutely necessary, compressed air (from a safe distance) can be used to remove loose dust. Avoid contact with water and any chemicals. ## What to avoid Avoid contact with acids and other chemicals that may damage the mineral. As an arsenate, it is potentially toxic if ingested or if dust is inhaled, so caution should be exercised and hands washed after contact with the specimen. Avoid ultrasonic and steam cleaning. Protect from high temperatures and humidity. ## Storage Segerstromite specimens should be stored under stable conditions, in a closed "micromount" box, to protect them from dust, mechanical damage, and humidity. Avoid exposure to direct sunlight, although there is no evidence of it fading.