FeS2: A try to describe a species in the Lusatian Tertiary
The compound of sulfur and iron always concerned the peoples mind. As Fool’s Gold it cheated the inexperienced treasure hunter since ancient times.
In the early 20th century, FeS2 minerals serve as feedstock for the production of sulfuric acid and therefore the upcoming chemical industry. Beside the economic aspect of the deposit, today FeS2 is watched stronger under environmentally relevant points of view.
In the area of the Lusatian lignite district the cubic crystallizing pyrite, the orthorhombic marcasite and cryptocristalline FeS2, known as Melnikovite, can be found in tertiary units. Distinct effects of anisotropy in crossed polarized light distinguish the Markasite from his isotropic sibling, which, in geological context, originates in rather high formation temperatures.
The orthorhombic bipyramidal crystal class can be recognized at some larger crystals, like, for example, on joint surfaces of the 2. Lusatian seam (picture). Even partly very nice twinning can be observed.
A variety represents the framboise FeS2. Here, it is a matter of spherical aggregates up to 50 µm in diameter, composed of numerous bipyramidal, pyritical microcrystals in nanometer range. In literature this structure is explained by microbial origin and occurs in many sites of the stratigraphic sequence of the Lusatian Tertiary.
Beside these structures, recrystallization phenomena of the probably primary framboise marcasite can be observed. Under the microscope, they occur as joint filling or concretions. In the period of diagenesis, the presence of hollows, like cells in lignite timber, or open joint areas as well as diffusion processes in those hollows, play a decisive role.
The interesting thing beside the scientific point of view is, that the framboise phases have an essential bigger reactive surface than the recrystallized objects. This lead, under the influence of atmospheric oxygen, to a quicker and more effective oxidation and by association to a more intense acidification reaction of the affected sediments.
The formation of FeS2 is still an object of investigation at LAOP, and knowlegde constantly grows.