Remote Sensing Index • Soil & Clay Minerals

CLAY – Clay Minerals Ratio Index

The CLAY index (Clay Minerals Ratio) enhances clay-rich and hydrothermally altered rocks using shortwave infrared (SWIR) bands. It is widely used in geological mapping and soil studies to highlight clay minerals compared to surrounding materials.

1. Concept & Equation of CLAY

The CLAY (Clay Minerals Ratio) index relies on the diagnostic absorption features of hydrous clay minerals in the shortwave-infrared region. Clay and alunite-rich rocks absorb more strongly in SWIR2 than in SWIR1, so their ratio highlights clay-rich pixels.

General Equation

For broadband multispectral sensors (e.g., Landsat, Sentinel-2), a simple and widely used clay ratio is:

CLAY = SWIR1 / SWIR2 Dimensionless ratio

Where:

  • SWIR1 – shortwave infrared band 1 (around 1.55–1.75 µm)
  • SWIR2 – shortwave infrared band 2 (around 2.08–2.35 µm)

Typical Interpretation

CLAY Value (relative) Interpretation (context-dependent)
Low (≈ 0.8 or less) Weak clay signal, possibly sandy or non-clay materials
Moderate (~0.8 – 1.1) Mixed lithology or moderate clay content
High (> 1.1) Clay-rich or hydrothermally altered rocks, higher clay mineral content

Thresholds should always be calibrated with field data, reference spectra, or local geological information.

2. Data & Practical Use of CLAY

Common Sensors & Bands

  • Landsat 8/9 OLI
    • SWIR1: B6 (~1.6 µm)
    • SWIR2: B7 (~2.2 µm)
  • Sentinel-2 MSI
    • SWIR1: B11 (1610 nm)
    • SWIR2: B12 (2190 nm)

Typical Applications

  • Mapping hydrothermally altered rocks and clay caps
  • Supporting mineral exploration and lithological mapping
  • Soil studies where clay content is relevant to texture and fertility

Good Practice

  • Use surface reflectance products and mask clouds/shadows.
  • Apply CLAY mainly over exposed rocks or bare soil (vegetation & water can distort the signal).
  • Always validate with field data, geological maps, or lab spectroscopy where possible.
Soil & Geology Clay Minerals SWIR Ratio

3. Google Earth Engine Code – CLAY (SWIR1 / SWIR2)

Steps: open code.earthengine.google.com → New Script → paste the code → draw your AOI as geometry → Run → then export CLAY as GeoTIFF from the Tasks tab. 

// CLAY – Clay Minerals Ratio Index (SWIR1 / SWIR2)
// Example using Sentinel-2 SR in Google Earth Engine
// --------------------------------------------------
// 1) Go to: https://code.earthengine.google.com
// 2) Click "New Script" and paste this code.
// 3) On the map: draw your AOI (Polygon/Rectangle).
//    It will appear as a variable named 'geometry' in the left panel.
// 4) Click "Run" to display CLAY.
// 5) In the Tasks tab, click "Run" to export CLAY to Google Drive.

// -------------------------------------------------------
// 1. Define Area of Interest (AOI)
// -------------------------------------------------------
var roi = geometry;  // Make sure a 'geometry' object exists

// Center the map on the AOI
Map.centerObject(roi, 9);

// -------------------------------------------------------
// 2. Select Sentinel-2 Surface Reflectance collection
// -------------------------------------------------------
var startDate = '2021-01-01';
var endDate   = '2021-12-31';

// Simple cloud filter based on metadata percentage
var s2 = ee.ImageCollection('COPERNICUS/S2_SR')
  .filterBounds(roi)
  .filterDate(startDate, endDate)
  .filter(ee.Filter.lt('CLOUDY_PIXEL_PERCENTAGE', 20))
  .median()
  .clip(roi);

// -------------------------------------------------------
// 3. Compute CLAY index: SWIR1 / SWIR2
//    For Sentinel-2: SWIR1 = B11, SWIR2 = B12
// -------------------------------------------------------
var swir1 = s2.select('B11');
var swir2 = s2.select('B12');

var clay = swir1.divide(swir2).rename('CLAY');

// -------------------------------------------------------
// 4. Visualization
// -------------------------------------------------------
var clayVis = {
  min: 0.8,
  max: 1.3,
  palette: [
    '#2c7bb6', // low
    '#ffffbf',
    '#d7191c'  // high (clay-rich)
  ]
};

Map.addLayer(clay, clayVis, 'CLAY (SWIR1 / SWIR2)', true);

// Optional: True color for context
var rgb = ee.ImageCollection('COPERNICUS/S2_SR')
  .filterBounds(roi)
  .filterDate(startDate, endDate)
  .filter(ee.Filter.lt('CLOUDY_PIXEL_PERCENTAGE', 20))
  .select(['B4','B3','B2'])
  .median()
  .clip(roi);

Map.addLayer(rgb, {min:0, max:3000}, 'True Color (RGB)', false);

// -------------------------------------------------------
// 5. Export CLAY as GeoTIFF to Google Drive
// -------------------------------------------------------
Export.image.toDrive({
  image: clay,
  description: 'CLAY_Export',
  fileNamePrefix: 'CLAY_SWIR1_SWIR2',
  region: roi,
  scale: 20,            // Sentinel-2 SWIR bands are 20 m
  crs: 'EPSG:4326',
  maxPixels: 1e13
});

// End of CLAY index script