• John S. Wah

X-Ray Diffraction in Archaeology.

X-ray diffraction (XRD) is a laboratory technique used to identify minerals based on their crystal structure. XRD has many possible applications within archaeology. In XRD, a sample is bombarded with characteristic x-rays. The angle at which those x-rays are reflected is a function of the spacing of planes within the crystal lattice and is used to identify the minerals. X-ray diffraction can be applied to cultural materials recovered from archaeological sites, to features, and to the sediments and soils of an archaeological site.

XRD can be used to link pre-contact ceramics recovered at archaeological sites to the clay source areas where people extracted the raw material based on the mineralogy of both the clay sized (< 2 μm) particles and larger particles, inadvertently or intentionally included in the clay body, provided the source soils are unique or limited in distribution. For example, Jackland Series soils are rich in smectitic clay minerals and form in diabase, basalt, and gabbro on the northern Piedmont in Maryland and Virginia. Ceramics made of clay from a Jackland Series soil might be identified based on the high amounts of smectite in the clay fraction or from the presence of diabase or other parent rock particles in the silt or sand fraction. Additionally, Jackland Series soils have common iron-manganese concretions that can be distinctive.

Analysis of clay minerals and iron oxides by XRD can also be used to address suspected burn features like hearths and roasting pits on archaeological sites. Heating of clay minerals alters their structure. Smectites collapse irreversibly after heating beyond a certain temperature while in other clay minerals, distances between structural planes (d-spacing) decrease when heated. Iron oxides are also altered by heat with magnetite, goethite, and ferrihydrite transformed to maghemite and hematite. Alteration of clay minerals and iron oxides in a feature can be a strong indicator of burning.

Where XRD might play its greatest role is in the characterization of the soils and sediments of an archaeological site. Soils and sediments, including their mineralogies, can be used to understand landscape evolution and reconstruct environments that influenced behavior. Along the Chesapeake Bay in Maryland, mineralogy of fine silt-sized particles (2-20 μm) shows a homogeneity in the 12,900 to 11,600 year old Younger Dryas loess (Figures 1 & 2). Over a contrasting parent material (eg. Schist bedrock on the MD Piedmont) mineralogy would identify the archaeologically significant yet often overlooked loess deposit, which has the potential to bury and preserve Paleo-Indian and earlier sites.

Figure 1. Younger Dryas loess from the western Delmarva Peninsula in Maryland.

Mineralogy is indicative of landscape stability as minerals in the soil weather and transform on stable landscapes over time. Mineralogy can also be used to identify stratigraphic markers and corroborate dates if allogenic materials (eg. volcanic ash) tied to dated events in other areas are identified in a horizon.

Lastly, clay mineralogy can help identify natural site disturbances resulting in mixed archaeological components. Clays with low layer charge are subject to shrinking and swelling with drying and wetting and ‘self mixing’, which can result in the displacement of artifacts. Vertisols, soils with high smetitic clay contents, can be particularly problematic due to the vertical movement of artifacts throughout the soil profile.

X-ray diffraction can play a significant role in archaeological investigations. It can be used as a primary tool for addressing research questions (e.g. identifying source areas for precontact pottery); as a method to test or support interpretations and/or other archaeological evidence (e.g. determining whether soils have been heated by fire and corroborating dates); to help interpret environments; and to identify soils that might be naturally problematic for archaeological site preservation and interpretation (e.g. smectite rich soils mixing archaeological components). While XRD many not be appropriate to use on every archaeological site, it’s another tool available to help explain the past.

Figure 2. X-ray diffraction pattern showing the mineralogy of fine silt fraction (2-20 μm) of the Bt horizons formed in Younger Dryas loess from sites across the western Delmarva. Q=quartz, K=kaolinite, M=mica, F=feldspar.

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