I have done a number of reflectance spectroscopy studies using both a PIMA and more recently the Terraspec to study alteration minerals for epithermal deposits and geothermal systems. Both have smectite, illite-smectite, illite with variable chlorite, calcite and kaolinite. In these studies I have calculated various spectral parameters derived from both reflectance and hull quotient corrected / continuum removed spectra. Curiously, the calculated H2O/Al-OH depth ratio  for the epithermal hosted in andesite for reflectance spectra show a better match with XRD results then the hull. However the opposite seems to be the case for the mostly rhyolite hosted geothermals. The shape of the hull differs between the two and it appears that for andesite there is typically a downward slope between 1900 and 2200 nm, whereas for the geothermal this interval is fairly flat / though the overall profile from 1200 to 2500nm is often convex. What is the cause of the continuum shape?

I should add, that the andesites are typically intensely altered and thus pale cream, white to pale - medium green / depending on chlorite content. Altered rhyolite from the geothermal shows the same range of colours.