A Laboratory Manual for X-Ray Powder Diffraction
The presence of organics, which causes a broad hump on X-ray powder diffraction patterns, can obscure the diffraction maxima of mineral species. Unfortunately, just about any treatment to clays involves some risk. With hydrogen peroxide, the danger is oxidizing octahedral iron and changing the layer charge. This is most likely with chlorite and vermiculite. However, one has to do something to resolve clay peaks in some organic-rich samples. The general rule (if there is one) is to go ahead with the peroxide treatment, but if you see some strange diffraction patterns that defy interpretation, suspect that iron oxidation has occurred. This is rarely the case under normal circumstances.