Plot of absorbance depths extracted from curves shown in figure 3 and similar mixtures using plagioclase feldspar and augite as the anhydrous phase. Solid lines represent basalt, dashed lines are feldspar, and dash-dot lines are augite. Palagonite = '*', bentonite='+', gypsum='o', and goethite='x'. Horizontal line at 0.56 represents the absorbance for Isidis Planitia published in Erard et al. [1991]. Representative error bars are shown on the gypsum/basalt mixture.