LAURITE
Laurite – RuS2 – is a rather rare sulfide, characteristic of ultramafic intrusions containing platinum-group elements. It is an ore of Ru.
It is classified within the Pyrite Group, in the Ehrlichmanite-Laurite Series. It may contain Os, Rh, Ir, and Fe.
1. Characteristics
Crystal system: Cubic diploidal.
Color: Black.
Habit: Crystals as octahedrons, cubes, and pyritohedrons, up to 1 mm in size. Rounded grains.
Cleavage: {111} perfect.
Tenacity: Very brittle.
Twinning: No.
Fracture: Subconchoidal.
Mohs Hardness: 7 – 7.5
Parting: No.
Streak: Dark gray.
Lustre: Strong metallic.
Diaphaneity: Opaque.
Density (g/cm³): 6.43
2. Geology and Deposits
Laurite occurs in cumulate deposits of ultramafic intrusive igneous rocks. A characteristic occurrence is in the hortonolite-dunites of the Merensky Reef in the Bushveld Complex (South Africa).
It forms euhedral crystals or occurs as inclusions in other minerals, such as chromite and spinel.
It occurs in sedimentary placers derived from ultramafic rocks, in the form of rounded black grains.
3. Mineral Associations
It occurs with common sulfides such as pyrite, chalcopyrite, pentlandite, pyrrhotite, and millerite.
It associates with minerals typical of ultramafic rocks containing Platinum-Group Elements, such as spinel, braggite, chromite, sperrylite, cooperite, kashinite, native platinum (polyxene variety), rutheniridosmine, isoferroplatinum, osmium, irarsite, ruthenium, rhodarsenide, hollingworthite, and native gold.
Also with chlorite (clinochlore) and limonite.
4. Transmitted Light Microscopy
Not applicable, as laurite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Polishing laurite is extremely difficult due to its high hardness and brittle nature. The resulting polish is even poorer than that of pyrite, cassiterite, or sperrylite. Its polishing hardness exceeds that of any other sulfide, including pyrite and sperrylite. Its hardness is also greater than that of chromite.
PLANE POLARIZED LIGHT – PPL
Reflection color: White to very light gray.
Compared to the color of sperrylite, the color of laurite is very similar.
Compared to the color of hollingworthite, the color of laurite is slightly darker – also gray – and very similar.
Compared to the color of kashinite, the color of laurite is darker.
Pleochroism: No.
Reflectivity: 38.73%; it changes with variations in chemical composition.
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic.
Internal reflections: No.
May be confused with: No information available.
General Characteristics:
Grain shape: idiomorphic crystals, which can appear as cubes, octahedra or pyritohedra (pentagonodecahedrons), measuring up to 1 mm. It occurs as diamond-shaped inclusions in other minerals, such as chromite and spinel.
Substitutions: Laurite can be replaced with native platinum.
