Turneaureite
Chemical formula: Ca₅(As⁵⁺O₄)₃Cl
Turneaureite is a rare calcium arsenate from the apatite group, occurring as colorless to grayish-white, hexagonal crystals.
Properties
- Mohs hardness
- 5
- Luster
- Vitreous to Resinous
- Streak
- White
- Density
- 3.60
- Fracture
- Irregular/Uneven
- Transparency
- Translucent
- Crystal system
- Hexagonal
Diagnostic features
## Identification Turneaureite is most easily identified by its hexagonal crystal form, hardness (5 on the Mohs scale), and occurrence in a specific geological environment – metamorphosed zinc or manganese ore deposits. A white streak and a density of approximately 3.60 g/cm³ are also important features. However, definitive identification requires advanced analytical methods, such as chemical analysis (EDS) or X-ray diffraction (XRD). ## Distinguishing from similar minerals Turneaureite is visually almost impossible to distinguish from other colorless or white minerals of the apatite group, such as fluorapatite, chlorapatite, or hydroxylapatite, without specialized testing. It can also be confused with some forms of quartz or calcite, but it differs from them in hardness and crystal form. ## Crystal forms Turneaureite crystals are typically hexagonal, prismatic in habit, often terminated by flat pinacoids. It also occurs as granular aggregates and massive fillings.
Geological environment
## Genesis Turneaureite is a mineral of metamorphic origin. It forms under conditions of high-grade metamorphism in calcium- and arsenic-rich rocks. It is most commonly found in metamorphosed, stratiform zinc ore deposits (as in Balmat, USA) or in iron and manganese deposits (as in Långban, Sweden). ## Mineral associations This mineral co-occurs with other minerals typical of its formation environment. In US localities, these include calcite, diopside, tremolite, fluorite, titanite, graphite, and pyrite. In Långban, Sweden, it is accompanied by hematite, magnetite, hausmannite, andradite, and other rare arsenates. ## Localities The most important and simultaneously type localities for turneaureite are mines in the Balmat area, New York (USA), the Franklin deposit in New Jersey (USA), and the famous Långban deposit in Värmland (Sweden). These are the only confirmed occurrences of this mineral worldwide.
Rarity
Very rare
For collectors
## Quality criteria The most valued specimens by collectors are those with well-formed, sharp, and clearly visible crystals, especially those that contrast with the color of the host rock (e.g., embedded in a darker mineral). Crystal size is a key factor, as most are microscopic – specimens with crystals exceeding a millimeter are already considered valuable. A confirmed locality, especially from one of the three type localities, is also important. ## Popular localities All three known localities – Balmat (New York, USA), Franklin (New Jersey, USA), and Långban (Sweden) – provide specimens of collector's significance. Each of them is classic and highly valued in the world of mineralogy due to the occurrence of many rare mineral species.
Care and storage
## Cleaning Turneaureite specimens should be cleaned very carefully, using a soft brush to remove dust. Brief rinsing in distilled water is permissible, followed by thorough drying. Due to its arsenic content, avoid inhaling dust and wash hands after contact with the mineral. ## What to avoid Avoid contact with acids and other strong chemicals, which can damage the mineral. Do not heat it or subject it to ultrasound. Prolonged exposure to moisture is not recommended. ## Storage Specimens are best stored in closed display boxes or cabinets to protect them from dust and mechanical damage. Avoid storing in areas with high humidity.