If one wants to use a theory based on plane parallel layers, optimally the samples would be describable as layers.  But a particulate sample often looks a jumbled maze of particles of various sizes and shapes, showing  no structured pattern of any kind, and certainly not literally divided into distinct, identical layers. Even so, it is a tenet of  Representative Layer Theory that for spectroscopic purposes,  we may treat the complex sample as if it were a series of layers, each one representative of the sample as a whole.