Jakobssonite

Chemical formula: CaAlF<sub>₅</sub>

Jakobssonite is a rare calcium and aluminum fluoride, forming colorless, tabular crystals in volcanic fumaroles.

## Characteristics Jakobssonite is a mineral from the halide group, specifically a complex calcium and aluminum fluoride. It occurs as very small, colorless, tabular crystals, typically not exceeding 0.2 mm. These crystals often form aggregates or grow on other minerals in their environment. Due to its small size and rarity, it is a mineral known primarily in scientific circles and among specialized collectors. ## Physical Properties The mineral is characterized by a vitreous luster. It is transparent and colorless. Its Mohs hardness is approximately 3, and its density is close to 2.96 g/cm³. It exhibits perfect cleavage in one direction. ## History and Name Jakobssonite was discovered in volcanic exhalation products (fumaroles) of the Hekla volcano in Iceland. It was described and approved as a new mineral species by the International Mineralogical Association (IMA) in 2011. Its name honors the Icelandic geologist and volcanologist Sveinn Petur Jakobsson (born 1939), in recognition of his contributions to the study of the geology and mineralogy of Iceland, including the Hekla volcano.

Properties

Mohs hardness: 3
Luster: Vitreous
Streak: White
Density: 2.96
Cleavage: Perfect on {001}
Fracture: Uneven
Transparency: Transparent
Crystal system: Monoclinic

Diagnostic features

## Identification Identification of jakobssonite is impossible without advanced analytical techniques, such as X-ray diffraction (XRD) and chemical analysis (EDS/WDS). In collector conditions, identification relies solely on a label from a reliable source confirming previous scientific studies. ## Distinguishing from Similar Minerals It can be confused with other colorless secondary minerals found in fumaroles, such as ralstonite, gearksutite, or other rare fluorides. Visual differentiation is practically impossible and requires specialized research equipment. ## Crystal Forms Jakobssonite forms thin, tabular crystals with a hexagonal or rectangular outline. These crystals occur individually or form small, rosette-like clusters and aggregates.

Geological environment

## Genesis Jakobssonite is a mineral of volcanic origin. It forms through sublimation from volcanic gases in high-temperature fumaroles (above 600°C). It crystallizes directly from the gas phase on the surface of volcanic rocks. ## Mineral Associations This mineral co-occurs with other fluorides and volcanic exhalation products. In its type locality (Hekla volcano), it was found in association with hematite, ralstonite, and undefined aluminum-fluoride phases. ## Localities The only confirmed occurrence of jakobssonite in the world is its type locality – fumaroles on the lava field from the 1991-2000 Hekla volcano eruption in southern Iceland.

Rarity

Extremely rare

For collectors

## Quality Criteria Due to its extreme rarity and microscopic size, every analytically confirmed jakobssonite specimen is scientifically and collector-wise valuable. The most highly prized specimens are those where the crystals, despite their small size, are well-formed, isolated, and situated on a contrasting matrix. The reliability and precise designation of the locality are crucial. ## Popular Localities The only source of specimens is the Hekla volcano in Iceland. Collecting material in this location is restricted and requires specialized expeditions, making specimens virtually unavailable on the open collector market.

Care and storage

## Cleaning Jakobssonite specimens are extremely delicate and small. Mechanical cleaning is inadvisable and can lead to crystal destruction. If absolutely necessary, compressed air (from a safe distance) can be used to remove loose dust particles. ## What to Avoid Avoid contact with water and any chemicals, as the mineral may be soluble. Protect it from shocks, ultrasound, and high temperatures. Due to its fragility, avoid any touching. ## Storage Specimens should be stored exclusively in specialized, sealed "micromount" boxes that protect against dust, moisture, and mechanical damage. Display is not recommended due to the mineral's small size and sensitivity.

Sources

IMA Mineral List (CNMNC)