Turn oxide data into a trusted CIA score. Choose corrections then compare rocks confidently fast. Download results share tables and guide field sampling decisions.
Enter oxide weight percentages (wt%). Use CaO* options if carbonates or apatite may inflate CaO.
The Chemical Index of Alteration (CIA) estimates the relative loss of mobile cations during chemical weathering. It is computed from oxide weight percentages as:
Chemical Index of Alteration (CIA) summarizes how strongly a rock or sediment has been chemically transformed. It compares relatively immobile Al2O3 to mobile bases CaO*, Na2O, and K2O. Fresh feldspar rich materials often plot near 45–55, while intensely leached profiles can exceed 85. Because CIA is unitless and percentage scaled, it supports quick screening across many samples. In practice, CIA responds to climate, drainage, and residence time, so interpret it relative to parent composition. Plotting CIA on A–CN–K diagrams can highlight feldspar breakdown and clay formation pathways, and it helps compare horizons within a soil profile over time.
Total CaO can be inflated by carbonates or apatite, pushing CIA artificially low. This calculator provides options to estimate CaO* as min(CaO, Na2O), to correct CaO using P2O5 for apatite, to use total CaO when appropriate, or to enter a custom CaO* from petrographic or sequential extraction work. Consistency of the chosen correction is essential for comparative studies.
CIW excludes K2O and is useful when potassium metasomatism or variable K-feldspar abundance is suspected. Compare CIA and CIW together: a high CIW with a moderate CIA may indicate potassium addition, while both high values usually reflect strong depletion of bases. Always pair indices with mineralogy, grain size, and depositional setting for reliable interpretation.
Use normalized major oxides reported in weight percent from XRF, ICP, or wet chemistry. Verify totals are reasonable and that Al2O3, CaO, Na2O, and K2O are nonnegative. If carbonate is present, avoid the “use total CaO” option unless CaO has already been corrected. Document analytical uncertainty; small errors in low-oxide samples can shift CIA by several points.
Enter the oxides, select a CaO* method, then calculate CIA and CIW. Record the method in your report, export CSV for spreadsheets, and export PDF for field or lab notebooks. For multi-sample projects, apply the same settings and flag outliers for rechecking. Use CIA as an index, not a standalone diagnosis, and interpret trends across stratigraphy or profiles.
CaO* is the calcium oxide attributed to silicate minerals only. It excludes carbonate and apatite calcium that does not track feldspar breakdown. Using CaO* helps prevent CIA from being biased low in carbonate-rich or phosphate-rich samples.
Enter oxide values as weight percent (wt%), matching typical laboratory reports. CIA is ideally computed from molar proportions, but wt% inputs provide a practical, consistent approximation for screening when applied uniformly across a dataset.
Values above about 80 usually indicate advanced chemical weathering with strong depletion of Ca, Na, and K relative to Al. Interpret with mineralogy and setting, because intense leaching, recycled sediment, or clay-rich sources can also raise CIA.
Carbonate minerals add CaO without adding Al2O3, increasing the denominator and decreasing CIA. If carbonates are present, select a CaO* correction method or supply a custom CaO* value derived from petrography or geochemical correction.
CIW excludes K2O, so it is useful where potassium has been added or removed independently, such as K-metasomatism, variable mica content, or diagenetic K-fixation. Comparing CIA and CIW can highlight K-related overprints.
If totals differ due to loss on ignition or analytical reporting, normalize major oxides to 100% before calculating, or keep the same reporting style for all samples. Consistency matters more than the exact total for comparative trends.
| Sample | Al2O3 | CaO | Na2O | K2O | P2O5 | Example CIA* |
|---|---|---|---|---|---|---|
| Granite-like | 14.50 | 2.10 | 3.40 | 4.70 | 0.15 | 58.70 |
| Basalt-like | 15.00 | 10.50 | 2.60 | 0.80 | 0.20 | 71.43 |
| Shale-like | 18.00 | 1.20 | 1.00 | 3.20 | 0.10 | 77.59 |
*Example CIA uses the min(CaO, Na2O) CaO* estimate for consistency.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.