Foggite
Chemical formula: CaAlPO<sub>4</sub>(OH)<sub>2</sub>·H<sub>2</sub>O
Foggite is a rare phosphate mineral, forming colorless or white, platy crystals in granitic pegmatite deposits.
Properties
- Mohs hardness
- 3.5
- Luster
- Vitreous
- Streak
- White
- Density
- 2.65
- Cleavage
- Perfect on {010}
- Fracture
- Uneven
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Foggite is difficult to identify visually without magnification. Characteristic features include very small, colorless or white, platy crystals forming radial or rosetted aggregates. It occurs in the specific environment of granitic pegmatites, often in association with other rare phosphates. ## Distinguishing from Similar Minerals It can be confused with other secondary phosphate minerals such as wardite, crandallite, or wavellite. It differs from wavellite in crystal habit (platy instead of acicular). Final distinction from other white or colorless phosphates requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS). ## Crystal Forms Crystals are tabular or platy, flattened parallel to {010} and elongated along [001]. They typically form radial, spherulitic, or rosetted aggregates, as well as thin coatings.
Geological environment
## Genesis Foggite is a secondary phosphate mineral. It forms through hydrothermal processes in the late stages of crystallization of complex granitic pegmatites. It crystallizes in vugs and fractures from solutions enriched in phosphorus, aluminum, and calcium. ## Mineral Associations This mineral co-occurs with other pegmatite phosphates. At the type locality (Palermo No. 1 Mine), it was found in association with siderophyllite, quartz, brazilianite, whitlockite, wardite, eosphorite, and apatite. In other localities, it is accompanied by, among others, crandallite, montgomeryite, and zircon. ## Localities The most important and confirmed occurrences of foggite are: - **USA**: Palermo No. 1 Mine in North Groton, Grafton County, New Hampshire (type locality); Foote Lithium Co. mine in Kings Mountain, Cleveland County, North Carolina. - **Brazil**: Sapucaia Mine in Sapucaia do Norte, Minas Gerais.
Rarity
Very rare
For collectors
## Quality Criteria The collector appeal of foggite primarily depends on the quality and richness of its aggregates. Specimens with distinct, well-formed, radial or rosetted crystal clusters, contrasting with the rock matrix, are most highly valued. Due to the microscopic size of the crystals, specimens are evaluated under magnification. Purity (lack of damage) and good association with other rare pegmatite minerals also increase a specimen's value. ## Popular Localities The most classic and sought-after foggite specimens by collectors come from its type locality – the Palermo No. 1 Mine in New Hampshire, USA. Material from this mine serves as the standard for the species.
Care and storage
## Cleaning Foggite specimens should be cleaned very carefully, using a soft brush to remove dust. Due to its rarity and delicate nature, avoiding contact with water is the safest approach. If liquid is necessary, only a very small amount of isopropyl alcohol should be used, applied locally. ## What to Avoid Avoid contact with water, acids, and other chemicals that can damage or dissolve the mineral. Foggite is soft and brittle, so it must be protected from impacts and scratches. It should not be heated or exposed to prolonged sunlight. ## Storage Foggite specimens are best stored in enclosed, padded "micromount" boxes to protect them from mechanical damage, dust, and moisture. They should be kept under stable room conditions.