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Genthelvite
Current inventory:  1 gem
 

Chemistry:  Zn₄Be₃(SiO₄)₃S

Discovered in 1944;   IMA status: Valid (pre-IMA; Grandfathered).
Genthelvite is named after Frederick August Ludwig Karl Wilhelm Genth (1820-1893), German-American chemist and mineralogist, who described a zinc-rich Helvite (which was later renamed Genthelvite); and its relationship to Helvite.

Classification

Mineral Classification:

Silicates

Strunz 8th Ed. ID:

8/J.12-30

Nickel-Strunz 10th Ed. ID:

9.FB.10

9 : SILICATES (Germanates)
F : Tektosilicates without zeolitic H₂O
B : Tektosilicates with additional anions
30 : Helvite Group, Danalite-Genthelvite Series, Danalite-Helvite Series, Genthelvite-Helvite Series

Crystal Data

Crystallography:

Isometric - Hextetrahedral

Crystal Habit:

Crystals commonly tetrahedral, up to 5 cm in diameter; also as large rounded grains

Twinning:

None

Physical Properties

Cleavage:

[111] Poor

Fracture:

Irregular/Uneven

Tenacity:

Brittle

Hardness (Mohs):

6.0 - 6.5

Density:

3.44 - 3.70 (g/cm³)

Luminescence:

Strong green fluorescence in LW and SW UV; short-lived phosphorescence

Radioactivity:

Not Radioactive

Other:

Gelatinizes with acids

Optical Properties

Color:

Red, Pink, Colourless, White, Yellow, pale Green to Emerald Green; darkens to Brown and Black on weathering

Transparency:

Transparent, Translucent

Luster:

Vitreous

Refractive Index:

1.738 - 1.752  Isotropic

Birefringence:

0.00  Isotropic

Dispersion:

n/a

Pleochroism:

None

Occurances

Geological Setting:

In miarolitic cavities in granite pegmatites; in alkaline granites and syenites, greisens, and skarns.

Type Locality:

West Cheyenne Canon, Cheyenne District (St. Peters Dome District), El Paso Co., Colorado, USA

Year Discovered:

1944

View mineral photos:

Genthelvite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Genthelvite is an extremely rare mineral and gem that forms a mineral series with Helvite and Danalite. Genthelvite is a member of the Helvine Group of minerals that also includes Danalite, Helvine and Tugtupite. Genthelvite was discovered in Cheyenne Canyon, El Paso County, Colorado, USA in 1944 and has since been found in several other locations around the world. Gems are usually very small, less than a quarter carat, and included. Colors include colorless, yellow, green, pink to red and may darken to brown and black from weathering.

The most notable locations for gem quality crystals are Poudrette quarry Mont Saint-Hilaire, Rouville County, Québec, Canada; Bakstevalåsen, Gunhildrud, Øvre Eiker, Buskerud, Norway; Zagi Mountain, Mulla Ghori, Khyber Agency, Federally Administered Tribal Areas (FATA), Pakistan; West Cheyenne Canyon, near St. Peters Dome, El Paso County, Colorado, USA. These locations produce only very small crystals.
 

  
Genthelvite gems for sale: