How are Bismuth minerals formed geologically and how does Bismuth formation relates to plate tectonics?
Subject: Discovery of Bismuth
Who discovered Bismuth?
Bismuth is an interesting yet fairly rare element in rocks on Earth.
It was first identified as an element in 1753 by Claude Geoffrey
the Younger. It's minerals were known earlier but were misidentified
as Tin or Lead ores.
Bismuth shares geochemical affinities with other heavy main group elements
in group 5 (Sb, As), as well as the group 4 elements Sn and Pb. Its
halogenated compounds are fairly volatile (as are halogenated compounds
of Pb and Po) and it is released fairly effectively to the gas phase
during volcanic eruptions (with an efficiency of degassing of about
20%). It has a number of short-lived radioactive isotopes that occur
naturally due to the decay of naturally-occuring U. One such isotope,
210Bi, has been used to examine processes occuring in volcanic aerosols,
along with 210Po and 210Pb.
Bismuth is actually present in only very small quantities in most rocks. It occurs in ore-quantities mainly in hydrothermal/geothermal veins as the mineral Bismuthite (Bi2S3). It is almost always found coexisiting with Stibnite (Sb2S3). These minerals are typically associated with sulfur-bearing minerals of other "chacophile" (sulfur-loving) main group (Pb) and/or transition metal (Cu, Zn, Hg) elements.
Hydrothermal deposits in general form from hot fluids of high water content (either derived from magmatic water or ground water that has interacted with a magma) as they force their way through cracks in the country rock. They typically contain minerals of chemical elements that are found in only very low quantities elsewhere. This is because these elements form ions that are usually either too massive or too highly charged to "fit" into the more common rock forming minerals. Hydrothermal deposits are found in all tectonic environments displaying recent or ancient volcanic activity (mid-ocean-ridge spreading centers, plate collision zones and intra-plate volcanoes). In general, plate collision ("subduction") zone volcanoes produce the most concentrated deposits containing Bi.
Dr. Ken Rubin, Assistant Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu HI 96822