WURTZITE
Wurtzite – (Zn,Fe)S – is a rarer sulfide, typical of hydrothermal veins, where it integrates polymetallic ores with zinc.
It is classified in the Wurtzite Group, is dimorphic with sphalerite, forms a series with buseckite ((Fe,Zn,Mn)S) and has three varieties.
1. Characteristics
Crystal system: Hexagonal, dihexagonal pyramidal
Color: Reddish-brown, dark, black, orange-brown.
Habit: Massive, crusty, fibrous, botryoidal. Crystals rare, tabular or hemimorphic pyramidal.
Cleavage: {11-20} very good, {0001} bad. Striations in {10-10} and {10-11}.
Tenacity: Brittle.
Twinning: Pseudocubic.
Fracture: Irregular
Mohs Hardness: 3.5 – 4
Parting: No.
Streak: Brown
Lustre: Resinous to submetallic
Diaphaneity: Transparent.
Density (g/cm³): 4 – 4.1
2. Geology and Deposits
Wurtzite is typical of Zn-rich hydrothermal veins, where it is one of the last minerals to form.
It also forms at low temperatures in concretions (“clay-ironstone”) in sedimentary rocks, occupying desiccation fractures.
3. Mineral Associations
It ocurrs associated with minerals typical of hydrothermal veins, such as quartz.
It is part of the sulfide group of these veins, along with galena, sphalerite (“Schalenblende”), pyrite, chalcopyrite, jordanite, marcasite, tennantite, franckeite, and cylindrite.
It is also associated with cassiterite, barite, and garnet (andradite).
4. Transmitted Light Microscopy
Refraction indices: nω: 2,356 nε: 2,378
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism:No information available.
Relief: Very high.
Cleavage: {11-20} very good, {0001} bad.
Habits:
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.022, corresponding to colors up to the end of the 1st order: gray, white, yellow, orange, red, and blue.
Extinction:
Elongation sign:
Twins:
Zoning:
CONVERGENT LIGHT
Character: U(+)
2V angle: No.
Alterations:
May be confused with: No information available.
5. Reflected Light Microscopy
Sample preparation: The behavior of wurtzite upon polishing is similar to that of sphalerite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Gray with a subtle bluish tint.
Compared to sphalerite, the color is very similar.
Pleochroism: No, but the relief in the section may simulate pleochroism.
Reflectivity: 16.26%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: No anisotropy is observed.
Internal reflections: Yellow to dark brown, very common, abundant.
May be confused with: Sphalerite, which is practically identical. It is usually impossible to differentiate sphalerite and wurtzite by reflected light. The absence of twinning indicates the presence of wurtzite. Sometimes a thin section helps in identification, but generally X-ray diffractometry is necessary.
General Characteristics:
Grain shape: very characteristic of wurtzite are radial aggregates (“ice-flower”), a structure that can be preserved when wurtzite alters to sphalerite. Concentric and banded textures are very common. Idiomorphic crystals are possible or aggregated in bundles.
Zoning, very well-developed. can be frequently observed.
Spectacular banding is frequent, where bands of wurtzite can alternate with bands of sphalerite, marcasite, pyrite, and sometimes galena.
Epitaxy with chalcopyrite is possible.
Cleavage parallel to (0001) can be clearly visible.
Twinning is not observed, which is diagnostic in relation to sphalerite, which frequently exhibits twinning. However, radial aggregates of wurtzite can simulate twinning.
Substitution 1: wurtzite can be substituted by pyrite, marcasite, and galena.
Substitution 2: wurtzite replaces teallite, galena calcite, and dolomite.
Transformation of wurtzite into sphalerite and vice versa occurs and is difficult to recognize.