Natropharmacosiderite
Chemical formula: NaFe<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
A rare sodium and iron arsenate with characteristic cubic crystals and an intense green color.
Properties
- Mohs hardness
- 2.5
- Luster
- Vitreous
- Streak
- Pale green
- Density
- 3.11
- Cleavage
- Good on {001}
- Fracture
- Uneven
- Transparency
- Translucent
- Crystal system
- Cubic
Diagnostic features
## Identification A key diagnostic feature of natropharmacosiderite is the occurrence of small, cubic crystals with an intense green color. Its genesis in the oxidation zones of ore deposits is also an important clue. However, final confirmation requires advanced chemical (EDS) or X-ray (XRD) analyses to distinguish it from other members of the pharmacosiderite group. ## Distinguishing from Similar Minerals Natropharmacosiderite is visually indistinguishable from other minerals of the pharmacosiderite group, such as pharmacosiderite (potassium variety) or barypharmacosiderite (barium variety). All form identical cubic crystals. It can be confused with other secondary arsenates, such as scorodite, but scorodite forms crystals of a different habit (orthorhombic) and rarely has such a regular, cubic habit. ## Crystal Forms This mineral crystallizes in the isometric system, forming almost exclusively crystals with a cubic habit. Crystals are usually small, reaching up to 1 mm, and form druses, coatings, and small, scattered aggregates on the surface of the host rock.
Geological environment
## Genesis Natropharmacosiderite is a secondary mineral, forming in the oxidation (weathering) zones of ore deposits rich in arsenic, mainly arsenopyrite. It forms under low-temperature conditions as a result of reactions between sodium-containing solutions and the oxidation products of arsenic and iron minerals. ## Mineral Associations It often co-occurs with other secondary minerals of the oxidation zone. Its most common associations include scorodite, jarosite, goethite, pharmacosiderite, arsenopyrite, pyrite, and quartz. ## Localities The most important and best-documented occurrences of this rare mineral are found in several places worldwide. Key localities include: the Gold Hill mine in Tooele County, Utah, USA (type locality); the Clara mine in the Black Forest, Germany; Lavrion in Greece; and also in Cornwall in the United Kingdom.
Rarity
Rare
For collectors
## Quality Criteria Specimens with well-formed, sharp cubic crystals of intense, emerald-green color are most valued by collectors. Contrast with the rock matrix and the richness of the druse are also important. The size of the crystals, although rarely exceeding 1 mm, is a significant factor – larger cubes are much more desirable. The purity of the specimen and the absence of damage to delicate crystals also increase its value. ## Popular Localities Specimens from the Gold Hill mine in Utah (USA) and the Clara mine in Germany are considered classic and most representative for this mineral. Material from these localities serves as a benchmark for natropharmacosiderite.
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, washing with water, especially with detergents, should be avoided. If it is necessary to remove loose contaminants, compressed air can be used from a safe distance. ## What to Avoid Contact with acids and other chemicals that can damage the mineral must be absolutely avoided. It is sensitive to high temperatures, which can lead to dehydration and structural change. As an arsenate mineral, dust generated during its handling or cleaning is toxic – inhalation should be avoided. ## Storage Collections containing natropharmacosiderite should be stored in stable conditions, away from sources of heat and moisture. It is best kept in a closed, padded display box to protect delicate crystals from mechanical damage and dust.