SELLAITE

    Class : Halides
    Subclass : Fluorides
    Crystal system : Tetragonal
    Chemistry : MgF2
    Rarity : Rare

Sellaite is a rare mineral. Its formation context remains subject to discussion, as this mineral is found in varied settings. It is known mainly in bituminous dolomites of cap-rocks, in evaporites and in volcanic fumaroles and blocks rejected by volcanoes. It is also reported in certain marbles and metamorphosed deposits of magnesite as well as in hydrothermal veins with fluorite. It was named in honor of Quintino Sella, Italian mining engineer and mineralogist. Sellaite is known in its occurrences most often in included microcrystals (more rarely in free geodic crystals). It occurs in colorless or white acicular prismatic crystals, sometimes gathered in fibroradiated aggregates. It is a very accessory ore of fluorine.

Main photo : Sellaite from Brumado, Bahia, Brazil © Rob Lavinsky

Sellaite from Clara Mine, Germany © Michael Wallenborn
Sellaite from Emmelberg, Eifel, Germany © Edgar Müller
Sellaite from Mount Bischoff Mine, Tasmania, Australia © Matthew Latham
Sellaite on quartz from Maine Mine, Saône-et-Loire, France © Pascal Ollic

Sellaite in the World

Beautiful transparent sellaite crystals are known, measuring up to 10 cm by 3 cm, and are twinned in a "V" pattern at the Brumado magnesite (Bahia, Brazil). It is also reported in the fluorite veins of the Clara mine (Germany). It is also reported in other contexts, for example the evaporite series of Bleicherode (Thuringia, Germany), as well as in the fumaroles and ejecta of metamorphic limestones of Vesuvius (Italy).

Sellaite in France

Originally, sellaite was discovered associated with native sulphur and celestite in moraines containing blocks of evaporite formations of the Alpine Triassic, at the Gebroulaz Glacier (Savoie, France). It was then found in abundance in the fluorite veins of Fontsante (Var), building radiate groups intercalated between the fluorite ribbons. It is also reported at the Maine mine (Saône-et-Loire).

Twinning

Twinning is known on {011}.

Fakes and treatments

No fakes listed for this mineral species.


Hardness : 5 to 5.5
Density : 3.15
Fracture : Conchoidal
Streak : White

TP : Transparent
RI : 1.378 to 1.390
Birefringence : 0.012
Optical character : Uniaxial +
Pleochroism : None
Fluorescence : Yellow-green

Solubility : Sulfuric acid, water

Magnetism : NoneRadioactivity : None