FIGURE 2. XRD scans for some of the K-exchanged amphibole-spiked Black Gold samples and the K-exchanged Libby sample, for each the longer 2 scan is a count time of 8 s and the shorter 180 s. (a) A subset of the spiked samples showing the 1000 ppm sample’s 110 peak that is our detection limit, and the relationship between the 110 peak area and added amphibole. (b) The top most scan is the bg_10000 ppm spiked sample with the unknown attic samples plotted in decreasing order of % amphibole. (c) A series of scans with differing count times for the bg_10000 ppm spiked sample, showing that at the 1% level, amphibole is detected by XRD at count times much less than the 180 s we used for our calibration method.