The Clay Minerals Ratio: Finding Alteration From Orbit
Clay and other hydroxyl-bearing minerals absorb light around 2.2 µm — deep in the shortwave infrared — and the clay ratio exploits that absorption to flag the alteration haloes that often ring ore deposits.
The idea behind it
The ratio rises where 2.2-µm absorption is strong, i.e. where clays, micas and other hydroxyl or carbonate minerals concentrate. These are exactly the minerals produced by hydrothermal alteration, so the ratio is a first-pass vector toward mineralised ground.
The formula
where SWIR1 is shortwave-infrared 1 reflectance (~1.6 µm), SWIR2 is shortwave-infrared 2 reflectance (~2.2 µm).
Reading the values
Elevated values flag clay-rich, hydroxyl-bearing hydrothermal alteration zones — a classic exploration vector.
Which bands to use
| Sentinel-2 | SWIR1 B11 · SWIR2 B12 |
| Landsat 8/9 | SWIR1 B6 · SWIR2 B7 |
In practice
It is a staple of reconnaissance mineral exploration, mapping argillic and phyllic alteration zones over large, poorly accessible terrains before boots ever hit the ground.
Where it struggles
It only works on exposed rock: vegetation, soil cover and even lichen mask the signal, and atmospheric correction is essential. The ratio flags a mineral family, not a specific mineral — treat hits as targets to verify, not conclusions.
Compute Clay Minerals on your own study area
Skip the code. Draw or upload a boundary and Spatial Research Suite runs Clay Minerals on live Sentinel-2 or Landsat imagery — with cloud masking, exports and citations built in.
Run this analysis in GISforus →Frequently asked
What does the clay minerals ratio detect?
Concentrations of clay, mica and other hydroxyl-bearing minerals with a 2.2-µm absorption feature, often associated with hydrothermal alteration.
Does vegetation affect the clay ratio?
Strongly — any canopy or soil cover masks the mineral signal, so it is used over well-exposed, arid terrain.