Greetings Abdelkader Ouali Mehadji,

Greigite and Smythite are often formed by microbial action during anaerobic methane oxidation coupled with sulphate reduction. Where Methane dominates or Sulphate is limited, these metastable environmental ferrimagnetic  Iron deficient Sulphide minerals can form and persist. Keep in mind that the type locality for Greigite is in the Mississippian St. Genevieve Limestone (J.V. Smith). If you have loose or crushed dried sample, you can use plastic jars and a hand magnet to concentrate the magnetic minerals. Inspection under a binocular microscope will reveal their textures. Often these FIS (fine grained Iron Sulphides) will be incompletely converted to pyrite and the 2 phases are optically distinctive by crystal habits (cubes versus framboids etc.) Since pyrite is diamagnetic, that it separates by hand magnet is entirely the FIS minerals. Past the optics, microprobe grain mounts or polished sections to get the chemistry. Often other unusual metals make it into the microbial sulphides: Mn etc, instead of Ni and Co as in high temperature iron sulphides. Finally the physical properties of the magnetization using hysteresis properties (eg. alternating gradient magnetometer, and thermomagnetic properties with Day Plot analysis). Good luck and the key issues to resolve are 1) that the magnetization is due to Greigite or other FIS and not Hematite and 2) that the FIS are low temperature microbial minerals like Greigite or Smythite rather than high temperature Pyrrhotite which is exclusively in hydrothermal ore deposits. Also, given most geothermal gradients, you likely have methane seepage and methane fueled diagenesis rather than gas hydrates as this is exclusively restricted to lower temperatures > ~7°C and higher pressures > ~100 atm. like permafrost and deep continental slope settings.

Dear T.C. Hamilton,

thanks very much for this precious information that could help me to caracterize this mineral.

T.S. not T. C. but you are welcome and I hope it helps. We struggled with this for some time before figuring it out. The literature is full of mis-identified fine grained magnetic iron sulphides. Troilite is only in Meteorites and the Mantle, not the bottom of Lakes! Pyrrhotite is a hydrothermal or epithermal skarn and VMS/SMS mineral with 2 different crystal structures but is not in environmental sediments at all!

Best greetings T. C. Hamilton

Thanks for the added precisions.

Note my name above. The S. is for Scott. C. is not my middle initial. Dr. T.C. Hamilton is a medical researcher not a Geologist/Geophysicist.

Cheers,

T. S. Hamilton