Example: Examining the Microstructure of Flint and Chert
Above: Chert, flint -- it's just all microcrystalline silica. Credit: John Vigay.
Microcrystalline silica -- chert, flint, jasper, novaculite, chalcedony -- was a desirable material for making stone tools. These rocks are all principally composed of quartz or its polymorphs (minerals with the same chemistry but different crystalline structure). They essentially exist on a continuum, so it is difficult to delineate geologically what is a "flint" versus what is a "jasper" unless one resorts to either regional terminology or "folk categories" based on color. What geologists term "jasper" (any high-iron microcrystalline quartz regardless of whether it is red, green, or black) differs from what archaeologists call "jasper" (reddish chert that may or may not be iron-rich). For the rest of this post, let's simply adopt the term "chert" to describe all microcrystalline-quartz rocks.
Another problem is that the impurities can be highly variable. This is obvious even to the naked eye -- chert is often spotty, blotchy, streaked, and/or banded, as seen in the photo above, and these color variations are typically due to variation in impurities and their concentrations. These variations result from its formation as a sedimentary rock -- a variety of sediments can accumulate over time, become cemented into the same rock, and weather differently. Not surprising, then, recent studies have found that sourcing chert based on color and trace elements is problematic. A more successful approach involves isotopic signatures, but there is another geological-based feature of chert which provides information about its history: grain structure or surface texture.
Below: Variation in surface textures and grain structures of four microcrystalline-silica rocks. These secondary-electron images come from Barbara Luedtke's book An Archaeologist's Guide to Chert and Flint, pp. 70-71. All four images have the same magnification and fields of view. From left to right these rocks are: light-gray novaculite from Magnet Cove, Arkansas; grey "wall flint" from Grimes Grave quarry, Norfolk County, UK; black Upper Mercer chert from Coshocton County, Ohio; and light-gray novaculite from Caddo Gap, Arkansas.
The adjective "microcrystalline" means that even the largest quartz grains are only 50 microns (0.05 millimeters) in diameter. The surface texture of chert, therefore, is too small to be seen without the aid of a microscope -- only the luster of chert (that is, how light reflects off its surface) provides a macroscopic indication of texture. This texture is strongly dependent on grain size, although it is hard to describe the texture objectively or quantitatively. Grain size determines the fracture mechanics and surface properties as well. Also important is that the grain size and surface texture can indicate how the chert was formed and altered by diagenesis.
It is hard to examine the so-called "microfabric" of chert using a visible-light microscope. Many grains in chert are micron-scale. A magnification of 1000X is required to make a 1-micron quartz grain appear 1-mm to the eye, and visible-light microscopes typically have a maximum magnification not much higher than that. Worse yet, standard petrographic thin sections are usually 30-microns thick, so one could see dozens of overlapping grains. Scanning electron microscopy, therefore, is often necessary for examining the grain structure.
Electron microscopy of different cherts reveal different grain structures. Folk and Weaver (1952) described grains ranging from "sharply defined equant polyhedral blocks" to "spongy" (502). This variation seems liked to how well-crystalized the grains are and whether the chert has been metamorphosed. A problem, though, is that no one has established yet a useful and non-subjective way to classify chert microfabrics. Until then, we are left to making our observations and trying to correlate them to the geological literature on quartz textures.
Below is a secondary-electron (SE) image of a Mousterian chert tool that came from the Anthropology Department collections. Unfortunately it is an example of what can be found in many old collections: this tool had one old label which read "Le Mousterian" and that was it. No other information was to be found. Consequently, I was allowed to borrow the tool and examine it using the microprobe. I had two main foci: I wanted to examine the grain size of the quartz, and I wanted to look for signs of use wear. You can see what the grains look like below:
Below: SE image of a Mousterian (Neandertal) chert tool; the field of view is about 800 x 800 microns.
Of course, I can't really come to any conclusions based on a single artifact, just make observations. I'll probably do more research along these lines when one of our archaeology professors returns from sabbatical. She hopes we'll identify signs of use wear on these stone tools as well as find residues on their surfaces. First, though, we need to establish the range of quartz grain structures that are typical for Mousterian tools.
For this work, I often consulted Barbara Luedtke's An Archaeologist's Guide to Chert and Flint, and much of what I discussed here can be found in her book. I also suggest with the following articles:
Bull, P.A. and A.W. Magee. 1988. Quartz Grains Studies: Environmental Reconstruction for Archaeologists by Scanning Electron Microscopy. In Scanning Electron Microscopy in Archaeology, edited by Sandra L. Olsen. BAR International Series 452, Oxford, England. pp. 103-116.
d'Errico, Francesco. 1988. A Study of Upper Paleolithic and Epipaleolithic Engraved Pebbles. In Scanning Electron Microscopy in Archaeology, edited by Sandra L. Olsen. BAR International Series 452, Oxford, England. pp. 169-184.
Folk, R.L. and C.E. Weaver. 1952. A Study of the Texture and Composition of Chert. American Journal of Science 250: 498-510.
Knutsson, Kjel. 1988. Chemical Etching of Wear Features on Experimental Quartz Tools. In Scanning Electron Microscopy in Archaeology, edited by Sandra L. Olsen. BAR International Series 452, Oxford, England. pp. 117-154.
12/2/07
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
