Acuminite

Chemical formula: SrAlF<sub>4</sub>(OH)·H<sub>2</sub>O

Akuminite is a very rare hydrated strontium aluminum fluoride, forming characteristic, sharp, arrow-head-like crystals.

## Characteristics Akuminite is a halide mineral, chemically classified as a hydrated strontium aluminum fluoride. It forms very characteristic, sharply terminated, almost perfectly formed crystals that resemble miniature arrowheads or blades. They are usually colorless or white and are small, rarely exceeding a few millimeters in length. They occur as single crystals or in small, loose aggregates. ## Physical Properties This mineral has a vitreous luster and is transparent to translucent. Its Mohs hardness is 3.5, making it relatively soft and susceptible to scratching. The density of akuminite is approximately 3.3 g/cm³, making it noticeably heavier than many common rock-forming minerals. ## Colors and Varieties Akuminite occurs almost exclusively in colorless or white forms. No colored varieties or trade names are known. ## History and Name The mineral was first described in 1987 by H. Pauly and O.V. Petersen. Its name comes from the Latin word *acumen*, meaning "sharp point" or "arrowhead," which directly refers to the typical, pointed shape of its crystals. ## Applications Due to its extreme rarity and small crystal size, akuminite has no industrial applications. It is solely an object of scientific interest and a prized acquisition for advanced collections of rare minerals.

Properties

Mohs hardness
3.5
Luster
Vitreous
Streak
White
Density
3.29 - 3.30
Cleavage
Good on {001} and {110}
Fracture
Conchoidal
Transparency
Transparent to Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification The most important diagnostic feature of akuminite is its unique, sharply terminated, "pointed" crystal shape, resembling an arrowhead. In combination with its occurrence (cryolite deposit) and mineral association, this feature allows for almost certain visual identification. ## Distinguishing from Similar Minerals Akuminite is sometimes confused with other rare fluorides from Ivittuut, such as pachnolite or thomsenolite. However, it is distinguished by its significantly sharper and more characteristic crystal shape. Pachnolite forms prismatic crystals, and thomsenolite often occurs in massive aggregates or in thicker, less distinct crystals. Final differentiation, however, requires advanced studies, such as X-ray diffraction (XRD). ## Crystal Forms Akuminite crystals are monoclinic but exhibit strong pseudorhombohedral morphology, adopting the form of sharp, double pyramids (bipyramids) or shapes described as pseudo-octahedral. They look like small, perfectly carved arrowheads.

Geological environment

## Genesis Akuminite is a secondary mineral, forming under very specific conditions in the oxidation and alteration zones of cryolite deposits. It forms as a result of low-temperature hydrothermal processes that modify the primary mineral composition of the deposit. ## Mineral Associations This mineral occurs in association with other, often rare, fluorides. It most commonly co-occurs with cryolite, celestine, fluorite, gearksutite, jarlite, pachnolite, thomsenolite, and weberite. ## Localities The most important and historically first locality for akuminite is the famous, now exhausted and inaccessible, cryolite deposit in Ivittuut (formerly Ivigtut) in Greenland. This is the type locality for this mineral. Specimens from this location are most valued by collectors.

Rarity

Very rare

For collectors

## Quality Criteria The collector's value of an akuminite specimen is primarily determined by the sharpness, clarity, and size of the crystals. The most desirable are sharp, undamaged, transparent crystals several millimeters long. Specimens where characteristic akuminite crystals grow on a contrasting matrix or occur in association with other rare minerals from the same locality are also highly valued. ## Popular Localities The only commercially and historically significant locality from which virtually all akuminite specimens available on the collector's market originate is Ivittuut in Greenland. Finding specimens from any other locality would be a mineralogical sensation.

Care and storage

## Cleaning Akuminite is a soft and brittle mineral, requiring very careful handling. To remove dust, it is best to use a soft brush or compressed air from a safe distance. If necessary, it can be rinsed in distilled water and then thoroughly dried. ## What to Avoid Avoid ultrasonic cleaners and mechanical cleaning, which can permanently damage delicate crystals. The mineral is sensitive to acids and other strong chemicals. As a hydrated mineral, it may be sensitive to sudden temperature changes and prolonged heating, which can lead to dehydration and structural damage. ## Storage Akuminite specimens are best stored in separate, padded membrane boxes or compartments to avoid contact with harder minerals that could scratch or chip them. It should be protected from dust, moisture, and direct sunlight.

External references

Sources

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