Mineral Mapping · Carbonate Rocks (ASTER TIR)

CARBONATE – Carbonate Index (CI)

The Carbonate Index (CI) is a thermal-infrared index defined for ASTER TIR bands to enhance carbonate-rich rocks (limestone, dolomite) using the emissivity contrast between band 13 and band 14.
مؤشر الكربونات (Carbonate Index) لتمييز الصخور الغنية بالكربونات مثل الحجر الجيري والدولوميت باستخدام نطاقات الحرارة الحرارية لأقمار ASTER.

1. Scientific Definition

The Carbonate Index (CI) exploits the diagnostic absorption features of carbonate minerals in the thermal infrared region. Carbonate rocks typically show lower emissivity (stronger absorption) around ~11.3 μm (ASTER band 14) and relatively higher emissivity near ~10.6 μm (ASTER band 13).

Formula

A widely used definition for ASTER TIR data is:

CARBONATE (CI) = Band13 / Band14 CI > 1.04 → high probability carbonate (low WV)

  • Band13 – ASTER TIR band centered around ~10.6 μm
  • Band14 – ASTER TIR band centered around ~11.3 μm

Typical Interpretation

CI (Band13 / Band14)Interpretation
< 1.00 Non-carbonate lithologies (silicic rocks, mafic rocks, most silicates)
1.00 – 1.04 Mixed pixels / possible weak carbonate signal
> 1.04 High carbonate content (limestone, dolostone) – under low water vapor

Thresholds such as CI > 1.04 are empirical and depend on atmospheric water vapor and scene conditions. They should be calibrated with field / lithologic data in each study area.

Main Applications

  • Regional mapping of carbonate formations (limestone, dolomite)
  • Exploration for carbonate-hosted ore deposits
  • Supporting lithologic classification from ASTER TIR data
  • Combining with Quartz Index (QI) and Mafic Index (MI) for lithologic discrimination

2. Data & Bands

ASTER TIR (Thermal Infrared)

  • Band 13: ~10.6 μm (TIR)
  • Band 14: ~11.3 μm (TIR)
  • Spatial resolution: ~90 m (native ASTER TIR)

Recommended Source in Google Earth Engine

  • NASA/ASTER_GED/AG100_003 – Global emissivity dataset (bands 10–14)
  • Uses mean emissivity per ASTER TIR band (2000–2008) at 100 m resolution

Best Practices

  • Use emissivity (not raw radiance) where possible for more robust mineral mapping.
  • Restrict analysis to sparsely vegetated / bare rock areas (low NDVI).
  • Mask water bodies and clouds prior to index computation.
  • Calibrate CI thresholds using known carbonate outcrops and geological maps.

Suggested Palette

[ "#0b1120", "#1f2937", "#4b5563", "#22c55e", "#a3e635", "#ecfeff" ]

3. Google Earth Engine Code – CARBONATE Index (CI = B13 / B14)

// CARBONATE Index (CI) using ASTER Global Emissivity (NASA/ASTER_GED/AG100_003)
// CI = emissivity_band13 / emissivity_band14
// This dataset represents mean emissivity (2000–2008) at 100 m resolution.

// 1. Define AOI
var roi = geometry;   // Draw AOI as 'geometry' in the Code Editor
Map.centerObject(roi, 7);

// 2. Load ASTER GED AG100 emissivity product
var asterged = ee.Image('NASA/ASTER_GED/AG100_003')
  .select(['emissivity_band13','emissivity_band14']);

// 3. Compute Carbonate Index (CI = B13 / B14)
var carbonate = asterged.expression(
  'b13 / b14',
  {
    'b13': asterged.select('emissivity_band13'),
    'b14': asterged.select('emissivity_band14')
  }
).rename('CARBONATE_CI').clip(roi);

// 4. Visualization
var vis = {
  min: 0.9,
  max: 1.1,
  palette: ["#0b1120", "#1f2937", "#4b5563", "#22c55e", "#a3e635", "#ecfeff"]
};

Map.addLayer(carbonate, vis, 'CARBONATE Index (CI = B13/B14)');

// 5. Simple carbonate mask (e.g. CI > 1.04)
var carbonateMask = carbonate.gt(1.04).selfMask();  // threshold can be tuned
Map.addLayer(
  carbonateMask,
  {palette: ['#22c55e']},
  'High Carbonate Probability (CI > 1.04)',
  false
);

// 6. Optional: mask out vegetated pixels using NDVI (if needed)
//    ASTER GED also provides mean NDVI band.
var ndvi = ee.Image('NASA/ASTER_GED/AG100_003').select('ndvi').multiply(0.01);
var bare = ndvi.lt(0.2); // keep low vegetation areas
var carbonate_bare = carbonate.updateMask(bare);
Map.addLayer(carbonate_bare, vis, 'CARBONATE (bare / low vegetation)', false);

// 7. Export CARBONATE Index as GeoTIFF
Export.image.toDrive({
  image: carbonate,
  description: 'CARBONATE_Index_ASTER',
  fileNamePrefix: 'CARBONATE_CI_ASTER',
  region: roi,
  scale: 100,          // ASTER GED spatial resolution
  crs: 'EPSG:4326',
  maxPixels: 1e13
});