Example: Identifying an Inclusion within Dacite
Some time ago I was knapping a bifacial point out of dacite. During this particular attempt, a strike removed a flake in an unexpected way. One can see the ripples in the photo below where the flake removal suddenly changed direction. When I looked closely at the ripples, I noticed an inclusion there, about a millimeter in each dimension. It turns out that this inclusion altered the crack propagation within the flake and sent the flake awry. I placed the entire flake within the electron microprobe to identify the inclusion that ruined my day.
An examination of the inclusion's morphology (using secondary-electron imaging) and its composition (using the energy-dispersive spectrometer, EDS) revealed that it was orthoclase feldspar, also known as potassium feldspar (or just simply K-spar by geologists). It is a common mineral in igneous rocks like dacite, so it was not unexpected. This was also expected because of the inclusion's shape and color. This inclusion was white or light gray, and orthoclase feldspar is white, gray, or pink. It was also roughly rectangular with right-angle corners, and K-spar has two cleavage directions at right angles. This same combination of electron imaging and X-ray microanalysis can be used to identify inclusions in other flakes, whether artifacts or simply more of my flintknapping mistakes...
Above: Three highly magnified secondary-electron images show the fracture surface and right-angle
cleavage of the orthoclase feldspar (Latin for "easily cleaved material") inclusion within the dacite flake
7/25/07
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
