MELNIKOVITE

    Class : Sulfides and sulfosalts
    Subclass : Sulfides
    Crystal system : Cubic
    Chemistry : FeS2
    Rarity : Uncommon to fairly common


Melnikovite is a colloidal iron sulfide usually classified for convenience among the varieties of pyrite (or sometimes greigite for some authors). But the case of melnikovite is not simple : its nature as a non-crystallized gel makes it, like opal, a body at the limit of the mineral world. Furthermore, as this unstable colloidal gel slowly evolves into marcasite or pyrite, its existence is considered hypothetical, or even false, in Anglo-Saxon mineralogical reference works. It is a supergene variety of pyrite quite common in low temperature hydrothermal veins where it seems to come from the reprecipitation in a reducing environment of iron sulfides. It was named in reference to its discovery location : the Miocene chalk of the state of Melnikov (Russia). The appearance of melnikovite is however characteristic, in zoned concretionary deposits or in yellowish spherules quickly passing to bronze then to reddish. Unlike marcasite, spherulites are never fibrous.

Main photo : Pyrite balls from Cap Blanc-Nez, Pas-de-Calais, France

Melnikovite in the World

Melnikovite is present in many deposits around the world  it is impossible to make an exhaustive list.

Melnikovite in France

In France, melnikovite is known from the La Touche mine (Ille-et-Vilaine) which provided spectacular centimeter-sized concretionary spherulites in which melnikovite alternates with marcasite, fibrous sphalerite and fibrous pyrite. It is also reported at outcrops of uranium veins in Bois Noirs (Loire, France).

Twinning

No twin known for this mineral species.

Fakes and treatments

No fakes recorded for this mineral species.



Hardness : 6 to 6.5
Density : 4.95 to 5.10
Fracture : Irregular to conchoidal
Streak : Black


TP : Opaque
RI : -
Birefringence : -
Optical character : -
Pleochroism : None
Fluorescence : None


Solubility : Nitric acid

Magnetism : NoneRadioactivity : None