Gehlenite - Encyclopedia

Class : Silicates
Subclass : Sorosilicates
Crystal system : Tetragonal
Chemistry : Ca₂Al(AlSi)O₇
Rarity : Quite common

Gehlenite is isomorphic with akermanite, with which it forms a continuous series and whose deposits it often shares. This calcium silicate is a mineral in carbonate rocks found in basic to ultra-basic volcanic rocks, limestones and dolomites transformed by thermal metamorphism, and certain calcic alkaline rocks and certain meteorites. Akermanite and gehlenite also crystallize in abundance in foundry and incinerator furnaces and are therefore essential constituents of metallurgical slag and bottom ash. It was named in honor of the German chemist Adolf Ferdinand Gehlen. Natural gehlenite occurs in short prismatic crystals with the appearance of pseudocubes modified by the octahedron, or in masses ; it is colorless to grayish, grayish green or brownish yellow to brown. Artificial crystals from foundries and incinerators have acicular to bacillary habits, very different from those of natural crystals.

Main photo : 20 cm gehlenite from Vata de Sus, Vata de Jos, Hunedoara County, Romania © Martins da Pedra

2.6 cm gehlenite from Tin Mountain, Goldbelt District, California, USA © Rob Lavinsky

Gehlenite from Trieu Kaisin coal dump, Châtelet, Belgium © Alain Legrand

Gehlenite from Caspar Quarry, Ettringen, Germany © Stephan Wolfsried

Gehlenite from Monzoni Mountains, Trentino, Italy © Christopher O'Neill

Gehlenite in the World

Superb groups of centimeter-sized crystals are found at Predazzo and Val di Fassa in the Italian Dolomite Alps. Good crystals are also known from Crestmore (California) and several sites in Northern Ireland and Scotland. The world's largest crystals come from Vata de Sus (Romania) and measure up to 5 cm. It is also known in the Allende chondrite (Mexico).

Gehlenite in France

In France, gehlenite is known in the slag of Lapanouse (Aveyron) and in natural crystals on the Ouaki road near Saint-Louis (Réunion) and in the chondrite of Paris.

Twinning

No twin known for this mineral species.

Fakes and treatments

No fakes recorded for this mineral species.

Hardness : 5 to 6
Density : Undetermined
Fracture : Undetermined
Streak : White to gray

TP : Translucent to transparent
RI : 1.660 to 1.670
Birefringence : 0.010
Optical character : Uniaxial -
Pleochroism : Low
Fluorescence : None

Solubility : Hydrochloric acid

Magnetism : None
Radioactivity : None

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