Geology · Mineral Mapping

FERROUS – Ferrous Iron Index

The Ferrous Index highlights iron-bearing minerals by contrasting the NIR and SWIR reflectance. Used in lithology, mineral exploration, and alteration zone mapping.

1. Scientific Definition

The Ferrous Iron Index detects Fe²⁺-rich minerals such as biotite, hornblende, and magnetite by exploiting the strong SWIR absorption relative to the NIR channel.

Formula

FERROUS = (SWIR − NIR) / (SWIR + NIR)

  • SWIR — Shortwave Infrared band
  • NIR — Near Infrared band

Interpretation

  • > 0 → Fe-rich minerals (ferrous signatures)
  • < 0 → vegetation / low-iron rocks

Applications

  • Mineral exploration
  • Iron ore and alteration zone mapping
  • Geological lithological classification
  • Hydrothermal alteration studies

2. Required Bands

Sentinel-2

  • NIR → B8
  • SWIR → B11 (or B12 for deeper SWIR absorption)

Landsat 8/9

  • NIR → B5
  • SWIR1 → B6

Suggested Palette

[ "#0b1220", "#3a3f47", "#6b6f72", "#9c958c", "#d4c7b7" ]

3. Google Earth Engine Code – FERROUS


// FERROUS = (SWIR - NIR) / (SWIR + NIR)

var roi = geometry;
Map.centerObject(roi, 10);

// Sentinel-2 SR
var s2 = ee.ImageCollection("COPERNICUS/S2_SR")
  .filterBounds(roi)
  .filterDate("2023-01-01", "2023-12-31")
  .filter(ee.Filter.lt("CLOUDY_PIXEL_PERCENTAGE", 20))
  .select(["B8","B11"]); // NIR, SWIR

var img = s2.median().clip(roi);

// Compute Ferrous Index
var ferrous = img.expression(
  "(S - N) / (S + N)",
  {
    "S": img.select("B11"),
    "N": img.select("B8")
  }
).rename("FERROUS");

var vis = {
  min: -1,
  max: 1,
  palette: ["#0b1220","#3a3f47","#6b6f72","#9c958c","#d4c7b7"]
};

Map.addLayer(ferrous, vis, "FERROUS Index");

// Export
Export.image.toDrive({
  image: ferrous,
  description: "FERROUS_export",
  fileNamePrefix: "FERROUS_Index",
  region: roi,
  scale: 20,
  crs: "EPSG:4326",
  maxPixels: 1e13
});