Wurtzite
Wurtzite is a zinc iron sulfide mineral that is a less common form of Sphalerite . Wurtzite is trimorphous with Sphalerite and Matraite. Trimorphous means the three minerals have the same basic chemistry but different crystal structures. Opaque Wurtzite is found layered with Sphalerite , Galena and Marcasite in Schalenblende . Polished Schalenblende cabochons are available from time to time. Wurtizite has also been found as very small red-brown crystals at the Agios Philippos Mine, Évros Prefecture, Thraki Department, Greece. These crystals are very attractive as mineral specimens but too small for faceting. Wurtzite was not available as faceted, transparent gems until relatively large, bright, gemmy crystals were found in the Winter of 2012 and brought to the market in early 2013. These crystals were found in a single pocket in the Tanzanite deposit in the Merelani Hills, south of Arusha, Tanzania. The crystals measure up to 2 cm in diameter and have a tabular shape with pyramidal faces. These remarkable crystals are considered to be the best of the species by all who have seen them. There were very few gemmy crystals found making faceted Wurtzite gems very rare.
Other localities: In the USA, from near the Thomaston Dam, Litchfield County, Connecticut; at the Leonard mine, Butte, Silver Bow County, Montana; at Frisco, Beaver County, Utah; and from the Joplin district, Jasper County, Missouri. From Rachelshausen, near Gladenbach, Hesse, Germany. At Liskeard, Cornwall, England. From Mežica (Mies), Serbia. At Príbram,CzechRepublic. In Romania, from Baia Sprie (Felsóbánya). At Quispisiza, near Castro Virreyna, Peru. In a number of mines in the Huanuni district, Oruro, and Chocaya, Potosí, Bolivia.
| Chemical Formula: | (Zn,Fe)S |
| Zinc Iron Sulfide | |
| Molecular Weight: | 96.50 gm |
| Composition: | Zinc | 60.98 % | Zn | ||
| Iron | 5.79 % | Fe | |||
| Sulfur | 33.23 % | S | |||
| 100.00 % |
| Crystallography: | Hexagonal – Dihexagonal Pyramidal |
| Crystal Habit: | Crystals to 1.5 cm, commonly hemimorphic pyramidal {5052} and {1011}; also short prismatic to tabular {0001}; typically striated horizontally on {1010} and {1011}. The polytypes show steepening of the pyramid as the repeat distance increases along {0001}. As concentrically banded crusts, fibrous or columnar. |
| Twinning: | None |
| Cleavage: | Easy on {1120}; difficult on {0001}. |
| Fracture: | Irregular/Uneven |
| Tenacity: | Brittle |
| Moh’s Hardness: | 3.5 – 4.0 |
| Density : | 3.98 – 4.08; Faceted gems from Merelani measured 3.78 – 3.85. |
| Luminescence : | May fluoresce orange-red under LW UV. Faceted gems from Merelani were inert under both long- and short-wave UV. |
| Radioactivity: | Not Radioactive |
| Color : | Deep reddish brown, orange-brown or dark brown to brown-black; yellow to dark brown internal reflections common in reflected light. |
| Transparency: | Transparent to Translucent |
| Luster : | Resinous, brilliant submetallic on crystal faces |
| Refractive Index: | 2.356 – 2.378 Uniaxial ( + ) |
| Birefringence: | 0.022 |
| Dispersion : | n/a |
| Pleochroism: | Very weak, almost invisible |
| Internal reflections: | Yellow to dark brown |
