CALCINATION OF CALICHE FROM EASTERN NEW MEXICO: PRELIMINARY XRD RESULTS AND ARCHAEOLOGICAL IMPLICATIONS

The Llano Estacado of Eastern New Mexico is underlain by the Ogallala Formation which contains several caliche horizons, including the resistant "caprock". Caliche was the only important lithic material locally available to prehistoric people. When caliche is heated, it changes color from pale off-white shades to shades of gray. Traditionally, archaeologists have used this thermal discoloration to identify burnt caliche in the field and thereby locate cultural sites. Ethnographic studies report that fire temperatures of up to nearly 1000°C have been documented.

This study looked at the mineralogical changes in caliche as it is heated to progressively higher temperatures by using powder XRD to monitor the conversion of calcite to lime. Samples were collected along a drainage ditch (DD) within the Portales city limits, from a caliche pit (CP) south of Portales, and from the Blackwater Draw Locality No. 1 archaeological site (BW) located north of Portales.

Sample colors vary from white to pinkish-white and pinkish-gray. Upon heating, the DD and CP samples turned gray at <700°C; lighter shades of gray at 700° to <750°C; and pure white at 750°C and above. The BW samples showed no appreciable change in color which may reflect a difference in chemistry. This implies that not all caliche will change color upon heating. Integrated peak intensity of the calcite (104) reflection and the lime (200) reflection were used to monitor the conversion of calcite to lime. Preliminary results indicate that calcination begins at about 625°C for the BW and CP samples and 665°C for the DD samples. Conversion of calcite to lime is complete at 725°-750°C for BW and CP samples and closer to 800°C for the DD samples. Volatile loss was approximately linear during calcination and plateaued when all the calcite was converted to lime. Additional data will allow for the construction of calibration curves to determine the approximate temperature of exposure of cultural caliche samples.