MINERALOGY OF SOAPSTONE: PIEDMONT, SC AND BLUE RIDGE, NC

Soapstone is a medium-grade metamorphic rock composed mainly of talc and chlorite with variable amounts of amphibole, FeTi oxides and magnesite/dolomite. Native Americans used soapstone to form vessels during the Archaic Period. Previous studies attempted to source soapstone artifacts to quarries using bulk-rock geochemical techniques. We propose that mineralogy, in particular the composition of minerals within the soapstone, may be distinctive between quarries thus providing another method to identify (artifact/soapstone) sources.

Metaultramafic rocks occur in the Blue Ridge and Piedmont provinces of the Southern Appalachian Orogen. Soapstone from one locality in the Blue Ridge of NC (Blue Rock Quarry (BRQ), 31Yc7) and one locality in the Piedmont of SC (Pacolet Quarry (PQ), 38SP12) were selected for study. Both localities show evidence of prehistoric quarrying. The quarries were selected for study because similarities in bulk composition (BRQ: SiO₂ = 51.9 to 54.7, MgO = 29.7 to 30.5 wt % compared to PQ: SiO₂ = 50.5 to 53.5, MgO = 24.0 to 25.5 wt %) made their distinction problematic.

Mineralogy of the two quarries is similar: soapstones are rich in the soft phyllosilicates chlorite and talc (typically more than 80 modal %). Lesser amount of amphibole and FeTi oxides also occur in soapstone from both quarries. Carbonate minerals are only found in the BRQ. Two of the PQ samples are much higher in amphibole than any of the BRQ samples. Differences in modal mineralogy distinguish many BRQ samples from PQ samples.

Electron microprobe analyses reveal clinochlore as the chlorite mineral in the soapstone samples. The chlorite contains 1-2 wt % Cr₂O₃, reflecting the ultramafic protolith. Chlorite from BRQ is relatively homogeneous with small variations in mg# (88.3 - 92.2) compared to PQ (mg# 80.9 - 93.5). The combination of modal mineralogy and chlorite composition are effective in distinguishing between these two geochemically similar quarries.