A reflectance spectrum is a curve of how much light a surface returns at each wavelength. Most of that curve carries little information. The useful parts are the dips, where reflectance falls because the material absorbed energy at those particular wavelengths. Those dips are absorption features, and they are the physical basis of material identification from orbit.
Why absorption happens
Absorption occurs when a photon carries exactly the energy needed to move a material into a higher energy state. Two mechanisms dominate in the reflected solar range.
Electronic transitions involve electrons changing energy levels, usually around transition metal ions such as iron. These produce broad features, often tens or hundreds of nanometres wide, concentrated in the visible and near infrared. The red colouring of iron oxides is an electronic process you can see with your eyes.
Vibrational transitions involve chemical bonds stretching and bending. The fundamental vibrations of most mineral bonds sit in the thermal infrared, but their overtones and combination tones fall in the shortwave infrared between roughly 1.0 and 2.5 microns. These features are narrow and sharply defined. Bonds involving hydroxyl, water and carbonate are especially productive: aluminium bound hydroxyl absorbs near 2.2 microns, magnesium bound hydroxyl nearer 2.3 microns, and carbonate near 2.34 microns.
Why this makes minerals identifiable
A feature is diagnostic when its position depends on the specific chemistry and crystal structure of the material, not just its general class. Because the exact bond geometry differs from mineral to mineral, the absorption centre shifts by a few nanometres between species that are chemically similar. Kaolinite, dickite, illite and alunite all absorb near 2.2 microns, but at measurably different centres and with different shoulder shapes.
Reading those differences requires two things. First, enough spectral sampling to resolve a feature that may be only twenty to fifty nanometres wide. Second, a way to separate the feature from the overall brightness and slope of the curve, since the same mineral in shade and in sun produces very different absolute reflectance. Continuum removal solves the second problem by fitting a hull across the shoulders of the feature and dividing the spectrum by it, leaving a normalised dip whose depth and centre can be compared directly against reference spectra.
Depth relates loosely to abundance and grain size. Position relates to identity. Position is the part worth trusting.