Turn oxide chemistry into actionable indices fast. Compare samples, spot alteration, and report confidently now. Export tables to share results with your team easily.
| Sample | SiO2 | Al2O3 | CaO | Na2O | K2O | Fe2O3 | MgO | TiO2 | MnO |
|---|---|---|---|---|---|---|---|---|---|
| Basaltic Example | 52.10 | 15.60 | 9.20 | 3.10 | 1.00 | 10.80 | 6.40 | 1.10 | 0.15 |
This calculator converts oxide weight percent into molar proportions, then derives compact indices that highlight alteration and bulk chemistry. CIA, CIW, and PIA emphasize feldspar breakdown, while ASI and A/NK frame alumina saturation. ICV summarizes compositional variability using either weight or molar basis for consistent comparison across datasets. For best practice, check that your major-oxide total is reasonable for the analytical method and apply a consistent normalization strategy before comparing across projects. Because iron may be reported as FeO or Fe2O3, ensure you are using a uniform convention; otherwise ratios such as Fe2O3/MgO and ICV can shift.
CIA uses Al2O3 relative to CaO*, Na2O, and K2O, scaled to 100. Values below 50 commonly reflect fresh or weakly weathered material, 50–60 low weathering, 60–80 moderate, and above 80 intense leaching. CIW removes K2O to focus on plagioclase and calcic phases, so CIW can rise when K remains mobile or scarce.
PIA corrects for K addition by subtracting K2O in both numerator and denominator. Low PIA often indicates limited plagioclase alteration, whereas higher values point to progressive transformation toward clay-rich assemblages. When Ca occurs in carbonates or apatite, use CaO* to avoid artificially low indices caused by non-silicate calcium contributions.
ICV is computed as (Fe2O3 + K2O + Na2O + CaO + MgO + MnO + TiO2) / Al2O3. Values below 1 suggest a more compositionally mature signal dominated by alumina-rich phases, while values above 1 imply mineralogic diversity and limited sedimentary recycling. Choose the molar option when you want a composition-focused metric less sensitive to oxide molecular weights.
Using the built-in basaltic example (SiO2 52.10, Al2O3 15.60, CaO 9.20, Na2O 3.10, K2O 1.00), the tool returns CIA 40.51%, CIW 41.68%, and PIA 39.94%, consistent with minimal weathering. ASI is 0.681 and A/NK is 2.523, indicating a metaluminous tendency. ICV on the weight basis is 2.035, suggesting compositional variability worth checking against mineralogy and analytical totals. When you export, the CSV preserves inputs and results, while the PDF formats the same table for reports and lab notebooks. for quick review.
1) What is CaO* and when should I use it?
CaO* represents silicate-bound calcium only. Use it when Ca is partly hosted in carbonates, phosphates, or salts, so indices are not biased by non-silicate Ca. If you are unsure, compare results with CaO and CaO* to see sensitivity.
2) Why are CIA, CIW, and PIA based on molar values?
These indices compare element proportions, not mass. Converting wt% to molar proportions removes molecular-weight effects, making Al, Ca, Na, and K comparable on an atomic basis. This improves consistency when samples differ in oxide mix or reporting style.
3) Can I apply the calculator to sediments and igneous rocks?
Yes, but interpret differently. In igneous rocks, indices can reflect primary mineralogy as much as alteration. In sediments, they more often track weathering and recycling. Always pair results with petrography, mineral modes, and depositional context.
4) My oxide total is far from 100. Is that a problem?
It can be. Large deficits or excesses may indicate volatiles, loss on ignition, analytical bias, or missing oxides. Consider adding key oxides, checking units, or using a consistent normalization approach before comparing samples. Avoid over-interpreting small differences.
5) Why does ICV change when I switch the basis?
Weight-based ICV is a quick screening ratio, while molar-based ICV compares atomic proportions. Because each oxide has a different molecular weight, converting to molar space shifts contributions slightly. Pick one basis and keep it consistent across a project.
6) Does exporting change rounding or the underlying results?
No. Exports use the same values shown on screen. The CSV stores inputs and the results table for spreadsheets, and the PDF formats the same table for reporting. If you need more precision, adjust the displayed decimals in the code.
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.