Genthelvite - Encyclopedia

Class : Silicates
Subclass : Tectosilicates
Crystal system : Cubic
Chemistry : Zn₄Be₃(SiO₄)₃S
Rarity : Uncommon

Genthelvite is a complex silicate of beryllium and zinc which forms two continuous series : one with danalite (ferriferous) and the other with helvine (manganiferous). It is a magmatic mineral that is found in granites and their pegmatites, in pneumatolytic veins and greisens, granites and alkaline syenites, more rarely in skarns. It was named in honor of the American mineralogist Frederick August Ludwig Karl William Genth who discovered a helvite rich in zinc. Genthelvite is very varied in color : colorless or white, pale yellow to gray-yellow and brownish yellow, emerald green to pale green, rarely pink-red, brown or intense blue. It forms tetrahedra of up to 5 cm, and irregular aggregates measuring up to 25 cm. It is associated with different minerals depending on its deposits : bertrandite, columbite, albite and topaz (granites and pneumatolytic veins), analcime and aegyrine (nepheline syenites).

Main photo : Genthelvite from Huanggang Fe-Sn deposit, Hexigten Banner, Chifeng City, Inner Mongolia, China © Kelly Nash

Genthelvite from Huanggang Fe-Sn deposit, Chifeng, Inner Mongolia, China © Jiang Bin

Genthelvite from Poudrette Quarry, Quebec, Canada © Rob Lavinsky

Genthelvite of Washington Pass, Washington, USA © Saul Krotki

Genthelvite from Poudrette Quarry, Quebec, Canada © Modris Baum

Genthelvite in the World

Magnificent genthelvite crystals, probably the largest known, were extracted from the alkaline syenites of Mont St-Hilaire (Canada) as well as the Huanggang mining complex in Inner Mongolia (China). Beautiful specimens were brought back from the alkaline syenite massifs of Narssaq (Greenland), from the Kola peninsula including Lovozero and Khibiny (Russia), and from the albitized columbite terms of the Bukuru alkaline complex, near Jos (Nigeria). Centimeter-sized intense blue crystals are known from El Criollo Mine (Argentina). In Europe, the best crystals come from Langesundsfjord in southern Norway.

Genthelvite in France

Genthelvite is not present in the French underground.

Twinning

No twin known for this mineral species.

Fakes and treatments

No fakes recorded for this mineral species, but very difficult to differentiate from helvite and danalite without chemical analysis.

Hardness : 6 to 6.5
Density : 3.44 to 3.70
Fracture : Irregular
Streak : White

TP : Translucent to transparent
RI : 1.738 to 1.752
Birefringence : 0
Optical character : None
Pleochroism : None
Fluorescence : Green

Solubility : Acids

Magnetism : None
Radioactivity : None

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