Andersonite
Chemical formula: Na<sub>2</sub>Ca(U<sup>6+</sup>O<sub>2</sub>)(CO<sub>3</sub>)<sub>3</sub>·5-6H<sub>2</sub>O
Andersonite is a rare, secondary uranium mineral, distinguished by its intense, bright green fluorescence under ultraviolet light.
Properties
- Mohs hardness
- 2.5
- Luster
- Vitreous
- Streak
- Light green
- Density
- 2.8 - 2.9
- Cleavage
- Perfect on {0001}
- Fracture
- Conchoidal
- Transparency
- Transparent to translucent
- Crystal system
- Trigonal
Diagnostic features
## Identification The most important diagnostic feature of andersonite is its exceptionally strong, bright green or yellowish-green fluorescence under UV light. In daylight, it can be recognized by its light green color and small, pseudocubic crystals forming coatings. ## Distinguishing from Similar Minerals Andersonite is sometimes confused with other secondary green uranium minerals, such as liebigite, bayleyite, or schröckingerite. Liebigite and bayleyite have similar fluorescence but crystallize in the monoclinic system, forming different crystal habits (bladed, acicular). Schröckingerite also fluoresces strongly but is water-soluble and often forms rosette-like aggregates. ## Crystal Forms Andersonite crystallizes in the trigonal system, but its crystals almost always have the form of pseudocubic rhombohedra. It occurs as small, individual crystals scattered on the rock matrix, as well as in the form of granular aggregates, coatings, and thin crusts.
Geological environment
## Genesis Andersonite is a secondary mineral, forming in the oxidation zones of uranium deposits. It forms as a result of carbonate-rich waters acting on other, earlier uranium minerals. It is most often found as coatings and crusts on mine workings walls, in uranium and copper mines. ## Mineral Associations It often co-occurs with other secondary uranium minerals, such as gypsum, schröckingerite, bayleyite, liebigite, swartzite, as well as with uraninite (as a primary mineral). ## Localities The most important localities worldwide, from which well-formed specimens originate, are in the USA, especially in Arizona (Hillside Mine, Monument No. 2 Mine), Utah (mines in San Juan and Emery counties), and New Mexico. It is also known from Argentina (Mendoza province) and Slovenia.
Rarity
Rare
For collectors
## Quality Criteria The most valued specimens by collectors are those with sharp, well-formed, and transparent crystals of intense green color. Rich coatings covering a significant area of the host rock are also highly prized. Contrast with a dark rock matrix enhances the visual appeal of the specimen. The intensity of fluorescence is also key. ## Popular Localities Specimens from mines in Arizona and Utah in the USA are considered the best in the world. Specimens from the Hillside Mine (type locality) and the Monument No. 2 Mine in Arizona are classics and fetch high prices in the collector's market.
Care and storage
## Cleaning Andersonite is a very delicate mineral and is water-soluble. It should absolutely not be cleaned wet. To remove dust, a soft brush or compressed air from a safe distance can be used. ## What to Avoid Avoid contact with water, acids, and other chemicals that can damage or dissolve it. As a uranium mineral, it is radioactive and should be handled with caution – avoid inhaling dust and wash hands after each contact. It should not be stored in bedrooms or areas of permanent human presence. ## Storage Andersonite specimens should be stored in closed, airtight containers (e.g., "perky box" type) to protect them from dust, moisture, and mechanical damage. The container should be clearly labeled as containing a radioactive mineral.