Example: Unknown "Ball" from a Viking Site
Archaeologists recovered this unknown sphere in association with a Viking site. It looked metallic and felt heavy, though not as heavy as iron or lead. Nothing quite like this "ball" had been reported at Viking sites before. It seemed plausible that this object was the product of Viking metallurgy, but there were actually three possibilities:
(1) The object is artificial in origin and a product, either deliberate or not, of Viking metallurgy.
(2) The object is natural and is associated with the site to due its collection as a novelty by a Viking.
(3) The object is natural and not truly associated with the site, occurring nearby only as a coincidence.
Chemical analysis of the object could support either option #1 or options #2 and #3 -- it would be impossible to make a distinction between options #2 and #3 without further information about the archaeological stratigraphy and geological environment. When this object was brought to me, I did not have that additional information, so I could only determine whether the object was artificial or natural, whether it was human-made or geological.
My analysis using the electron microprobe revealed that the object was iron disulfide, FeS2, or pyrite. It is known also as "fool's gold" because it usually has a metallic luster and a pale yellowish-brassy hue. That color description fit this object, but pyrite normally forms isometric crystals and occurs as cubes. What caused the spherical shape? Did some Viking individual hammer a pyrite cluster into this shape? Not likely. Pyrite fractures conchoidally and is brittle, which means it would readily shatter when hammered. There was another answer instead.
Pyrite is typically found with other sulfide minerals in sedimentary rocks, metamorphic rocks, and quartz veins; in coal beds; and within fossils. Sedimentary rocks can form features known as concretions, where voids or pores within the rock is filled by some other mineral. Then if the filling mineral is harder than the surrounding sediment, weathering can wear away the host matrix and leave behind the concretion, frequently ovoid or spherical. Concretions can vary in size from microscopic to the 9-m concretions found in the Faiyum Depression in Egypt.
Iron sulfide concretions occur in various places around the world. One example are commonly known as Kansas pop rocks. Such concretions are found within layers of chalk and volcanic ash of Smoky Hill in Kansas. Their shapes are usually flat spheroids, and their sizes can be as large as 70 x 12 cm. They formed when iron sulfides precipitated within calcareous (calcium-carbonate-rich) marine sediments before lithification to chalk. Sandstone of the Ordovician Winnipeg Formation also contains abundant iron sulfide concretions, for instance.
This object, therefore, was a naturally occurring pyrite concretion. Consequently, the first possibility listed above (the "ball" is a product Viking metallurgy) was rejected while the other two possibilities were supported by the findings. The pyrite concretion could either (1) be associated with the Viking site to due its collection as a novelty or (2) not truly associated with the site, occurring nearby only as a coincidence. Without additional information about the stratigraphy and geological environment, we cannot say for sure which possibility is more likely.
5/30/07
Added:
Electron Microprobe Analysis in Archaeology
Electron microprobe analysis (EMPA), also known as electron probe microanalysis (EPMA), is an analytical technique that combines scanning electron microscopy (SEM) and compositional analysis using x-ray spectrometry. The ability to determine structure and chemistry of samples makes EMPA very versatile. This is a dominant analytical technique in geology, but it is not as commonly used in archaeology despite similar materials in studied both fields. Here I will post about topics in EMPA, artifacts I have analyzed, archaeological studies that use EMPA, etc. If there is a topic you'd like to see posted here, please let me know.
Ellery Frahm
Doctoral Candidate, Archaeology
Research Fellow, Geology & Geophysics
University of Minnesota - Twin Cities
