Here is a nice poliched Sphalerite sprecimen or so called Schalenblende from Poland.
Sphalerite or Schalenblende is not a single mineral but a banded mixture of various metallic sulfides. Its formation is partly driven by microbial activity in low-oxygen, hydrothermal environments. Sulfate-reducing bacteria are particularly involved, transforming dissolved sulfates into hydrogen sulfide, which reacts with metal ions to form sulfide minerals.
These processes produce colloidal sulfide gels in rhythmic cycles. Fine, jelly-like mixtures of tiny particles suspended in liquid, resulting in a milky appearance. As the gels precipitate, they form the rock’s characteristic banded structure. Schalenblende consists primarily of sphalerite (zinc sulfide), galena (lead sulfide), pyrite, and marcasite (iron sulfides), giving it its decorative yellow, brown, and black coloration.
It can be described as a sulfide microbialite. Kucha et al. (2010) demonstrate through micro- and nanostructural features in sphalerite-rich ores that microorganisms. Especially sulfate-reducing bacteria played a major role in ore formation. The authors interpret sphaleritic microglobules as fossilized bacterial mats, and sulfur isotope data further support this biogenic origin.
Although occasionally used as a gemstone, Schalenblende is only moderately suitable. Some components, such as pyrite and marcasite, may contain traces of heavy metals, and the rock is relatively brittle with a hardness of 3.5 to 4 on the Mohs scale. Nevertheless, it remains a popular collector’s item due to its unusual formation and vivid banded patterns. The described specimen comes from a well-known deposit in Poland.
Kersten, Carl (1844) Untersuchung der Schaalenblende von Raibel in Kärnthen. Annalen der Physik und Chemie, 139. 132-135 doi:10.1002/andp.18441390913
Kucha, H., Schroll, E., Raith, J. G., Halas, S. (2010) Microbial Sphalerite Formation in Carbonate-Hosted Zn-Pb Ores, Bleiberg, Austria: Micro- to Nanotextural and Sulfur Isotope Evidence. Economic Geology, 105 (5) 1005-1023 doi:10.2113/econgeo.105.5.1005
Leis, R. J., & Stinchcomb, B. L. (2015). Stromatolites: Ancient, beautiful, and earth-altering. Schiffer Publishing.
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