HERCYNITE
Hercynite – FeAl2O4 – is a relatively common oxide that occurs mainly as an accessory mineral in some types of igneous rocks. It has no economic importance.
Hercynite is one of the members of the Spinel Group and forms three series: with spinel, with gahnite, and with chromite. Like all members of the Spinel Group, it forms octahedral crystals.
Varieties of hercynite are picotite (a hercynite with Mg and Cr), chromopicotite (a variety of picotite (which is hercynite)), cobaltochromopicotite (a variety with Co of chromopicotite), and chromohercynite (with Cr).
1. Characteristics
Crystal system: Cubic hexaoctahedral.
Color: Dark blue-green, yellow, intense emerald green, or brown.
Habit: Octahedral crystals. Granular, massive, frequently found in magnetite as a demixing formation.
Cleavage: No.
Tenacity: Brittle.
Twinning: Spinel Law twins, in {111}, simple, multiple, and lamellar.
Fracture: Conchoidal.
Mohs Hardness: 7.5 – 8
Parting: On {111}, probable.
Streak: Dark grayish green to dark green.
Lustre: Vitreous.
Diaphaneity: Opaque, translucent at thin edges.
Density (g/cm³): 4.39
2. Geology and Deposits
Hercynite is a relatively common mineral that occurs as a secondary mineral in Ti-rich mafic and ultramafic igneous rocks, usually associated with magnetite and ilmenite.
In addition, it is found in ferruginous pelitic sediments subjected to high-grade metamorphism, such as cordierite gneisses and granulites.
It can occur detrital, in placers, in the heavy fraction of sediments.
3. Mineral Associations
It occurs associated with silicates such as sillimanite, cordierite, biotite, garnet (almandine), chlorite (clinochlore), andalusite, and sodalite.
It also occurs with other oxides such as magnetite, ilmenite, spinel, chromite, rutile, ilmenorutile, and corundum. In some cases, it occurs with native iron and troilite.
4. Transmitted Light Microscopy
Refraction indices: n: 1.80 – 1.83 (indices vary in the literature)
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Strong green color, without pleochroism.
Relief: Very high.
Cleavage: No.
Habits: Octahedral crystals, providing square and rhomboid sections.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Isotropic.
Extinction: Isotropic.
Elongation sign: Isotropic.
Twins: Isotropic.
Zoning: It may have a darker core.
CONVERGENT LIGHT
Character: Isotropic.
2V angle: Isotropic.
Alterations: No information available.
May be confused with: Few other minerals are green and isotropic. Among common minerals, few exhibit an intense green color (diopside, etc.), but all of these are anisotropic.
5. Reflected Light Microscopy
Sample preparation: Hercynite acquires a good polish, which requires some patience due to its higher hardness. Because there are variations in chemical composition, the polish varies between different “spinels”.
PLANE POLARIZED LIGHT – PPL
Reflection color: Dark gray.
Pleochroism: No.
Reflectivity: ~10%, similar to the reflectivity of the gangue.
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic.
Internal reflections: Abundant and intense, colorless, various shades of intense green, more rarely brown.
May be confused with: Spinel, since the characteristics under a microscope are the same.
General Characteristics:
Exsolution lamellae of magnetite may occur.