MULLITE

    Class : Silicates
    Subclass : Nesosilicates
    Crystal system : Orthorhombic
    Chemistry : PbCu(VO4)(OH)
    Rarity : Uncommon

Mullite is a rare aluminum silicate in nature. It is especially known in clayey xenoliths vitrified by lavas. Elsewhere, it is only observed in tiny patches in volcanic rocks, aluminous rocks subject to intense metamorphism or even diamondiferous kimberlites. On the other hand, it is formed in abundance in industrial furnaces by heating andalusite to 1450°C and kyanite or sillimanite to 1600°C : it is the basic material of the refractory industry. It is also used in the composition of various ceramics requiring high thermal resistance (automobile spark plugs for example). Calcined kaolin used for very white paints is a trade name which designates a mixture of mullite and cristobalite. The structure of mullite is close to that of sillimanite, the aluminum being partially replaced by silicon. It owes its name to its location of discovery : the Isle of Mull in Scotland. Mullite crystals are prismatic to acicular, colorless to pale pink, sometimes grayish.

Main photo : Mullite from Nickenicher Sattel, Eifel, Germany © Volker Betz

Mullite from Caspar Quarry, Eifel, Germany © Volker Betz
Mullite from Emmelberg, Eifel, Germany © Edgard Müller
Epitaxial mullite on hematite from Emmelberg, Eifel, Germany © Edgard Müller
Mullite from Caspar Quarry, Eifel, Germany © Michael Förch

Mullite in the World

Mullite is known in the xenoliths of the Isle of Mull (Scotland), the type locality, as well as in the volcanic rocks of Bellerberg (Germany) in millimeter crystals. It is also reported in the corundum gneiss of Sithean Sluaigh, still in Scotland, and in the sillimanite and kyanite gneiss of Val Sissone (Italy). Finally, it is found on the Kimberley diamond deposit (South Africa).

Mullite in France

In France, mullite is reported at Mont Denise near Puy-en-Velay (Haute-Loire).

Twinning

No twin known for this mineral species.

Fakes and treatments

No fakes recorded for this mineral species.


Hardness : 6 to 7
Density : 3.11 to 3.25
Fracture : Undetermined
Streak : White to gray

TP : Opaque to transparent
RI : 1.642 to 1.679
Birefringence : 0.012 to 0.026
Optical character : Biaxial +
Pleochroism : Visible
Fluorescence : None

Solubility : Insoluble

Magnetism : NoneRadioactivity : None