Example: Red Stone Tool -- Is It Pipestone or Shale?
Above: George Catlin's 1836 painting of pipestone quarries. Credit: Smithsonian.
In 2003, I was brought a thin section of a stone tool unearthed in Wisconsin. The stone flake was red in color, and a geologist hadn't been able to conclusively identify the material. This blade had been cut, and the thin slice had been mounted and polished. Examination with optical petrography was able to rule out a few materials (such as quartzite, jasper, and siltstone) but still left two fine-grained possibilities: shale and pipestone.
Shale is a sedimentary rock and variety of mudstone that is mostly clay and quartz. It is created when silt and clays deposited by streams and lakes are compacted. When subjected to heat and pressure, it undergoes metamorphism and becomes slate. When it weathers or undergoes another oxygen-rich process, shale turns red in color -- the red color comes from iron oxides in the form of hematite within the lithified clay matrix. It is usually about one-third clay, one-third quartz, and one-third other minerals, such as mica and feldspar.
Pipestone, also known as catlinite (named for the American painter George Catlin -- see above), is metamorphosed mudstone, so basically it is a slate. Because pipestone has been metamorphosed, its texture should be reflect that: miniscule silicate grains in shales are fused in slates. Pipestone originates between layers of Sioux quartzite and is exposed in only a few locations, such as Pipestone National Monument in southwest Minnesota. The name derives from its use by Native Americans to carve peace pipes from this read material.
I knew basically what to expect, but the mineralogy of pipestone isn't too well described in the literature. Therefore I followed a comparative approach -- I examined shale and pipestone and compared them to the artifact sample. The pipestone and artifact had similar low-grade metamorphic textures. The pipestone and flake artifact also had similar iron, potassium, and sodium contents. The artifact was less similar to the shale, although the relative proportions of silica and alumina seemed closer in the artifact and this shale. A large fraction, though, of pipestone is sericite mica, such as illite or muscovite, so there can be considerable variation in silica and alumina.
I concluded, therefore, that the artifact was probably made from pipestone (or a material very similar to it).
Below: BSE image and element maps for the artifact sample. The field of view is 300 x 300 microns.
Below: BSE image and element maps for shale. The field of view is 300 x 300 microns.
Below: BSE image and element maps for pipestone. The field of view is 300 x 300 microns.
11/26/07
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
