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Formula: CaAlF5
Fluoride of calcium and aluminium
Crystal system: Monoclinic
Specific gravity: 2.89 calculated
Streak: White
Colour: White
Luminescence: Does not fluoresce under long wave UV
Environments
Jakobssite forms crusts around fumaroles in active basaltic volcanoes.
Localities
There are two co-type localities, Hekla, Rangárþing ytra, Southern Region, Iceland, and the Eldfell Volcano, Heimaey
Island, Vestmannaeyjar archipelago, Vestmannaeyjar, Southern Region, Iceland.
At the type localities, jakobssonite was first found in crusts collected in 1988 from a fumarole on the Eldfell
volcano; it was subsequently found in similar crusts collected in 1991 from a fumarole on the Hekla volcano. Fumerole
temperatures were about 230ºC. Jakobssonite was found as acicular crystals up to 50 µm long, and also as a white
overgrowth on a yellowish base of massive hydrokenoralstonite,
anhydrite and jarosite which
together form a 2 to 3 cm thick crust on altered scoria. Associated
minerals include hydrokenoralstonite,
gypsum, hematite,
anhydrite and opal at the Eldfell
volcano. At the Hekla volcano associated minerals include leonardsenite,
heklaite, malladrite,
hieratite, fluorite and
chiolite
(MM 76.3.751–760, HOM, Mindat).
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