The Chemical Index of Alteration (CIA) after Nesbitt & Young (1982) is possibly the most widely accepted weathering index for rocks. In this article, you will learn how to calculate it, correct for non-siliciclastic CaO, as well as some things to keep in mind when interpreting the values and ternary diagram.
Background
The chemical weathering of siliciclastic rocks has a strong effect on the major element composition and the associated minerals. The composition of upper-crust rocks is dominated by plagioclase, quartz, K-feldspar, volcanic glass, biotite, and muscovite (Nesbitt & Young, 1984). With quartz being a stable mineral, the other components are rather unstable. The cations Ca, Na, and K are released during chemical weathering into weathering solutions.
The Chemical Index of Alteration (CIA) is a good measure of the degree of chemical weathering. High CIA values reflect the removal of labile cations, such as Ca2+, Na2+, and K+ in relation to more stable cations, such as Al3+ and Ti4+, from the rock during chemical weathering. Conversely, low CIA values suggest low weathering effects on these cations.
Nesbitt & Young (1982, 1984) used the general acid-based reactions (usually H2CO3) during weathering of feldspars and volcanic glass and investigated the kinetic behavior of these minerals. Feldspars commonly weather to kaolinite and illite, while mafic minerals and igneous glass weather to smectites and clay minerals (e.g. kaolinite and illite). They developed a framework of compositional changes of minerals during weathering, which can be used to predict weathering trends based on Al2O3, CaO, Na2O, and K2O. Furthermore, this is expressed in the calculation of the CIA, which in turn can be visualized in a ternary diagram (Figure 1).
How to calculate the Chemical Index of Alteration?
As explained above, the Chemical Index of Alteration (CIA) is an expression of the proportions of stable Al versus the labile Ca, Na, and K. The formula is as follows:
Chemical Index of Alteration (CIA)
CIA = [Al2O3/(Al2O3 + CaO* + Na2O + K2O)] x100
The oxide units in the CIA formula are in moles (not in wt.%!), and CaO* represents CaO in the siliciclastic fraction only.
Important
The Chemical Index of Alteration (CIA) is based on molecular proportions.
Therefore, in the first step, all oxides need to be converted into moles, and in the second step, CaO might require correction for non-siliciclastic CaO, e.g. from biogenic apatite and/or carbonates, such as calcite and dolomite.
Conversion of weight percent to moles
The CIA is based on molecular proportions. That means the oxides (usually expressed in weight percent, wt%) need to be converted into moles.
Al2O3 (moles) = Al2O3 (wt%) / 101.96128 (g/mol)
CaO (moles) = CaO (wt%) / 56.0774 (g/mol)
Na2O (moles) = Na2O (wt%) / 61.97894 (g/mol)
K2O (moles) = K2O (wt%) / 94.19600 (g/mol)
P2O5 (moles) = P2O5 (wt%) / 141.9445 (g/mol)
CO2 (moles) = CO2 (wt%) / 44.0095 (g/mol)
Corrections of CaO* for non-silicate CaO
The correction of CaO to CaO*, i.e., to the siliciclastic CaO only, is difficult and often impossible without reliable data for present carbonates and/or inorganic CO2. This is the weak point of the CIA (and other weathering indices relying on a corrected CaO). Nevertheless, there are correction attempts as follows:
1. Fedo et al. (1995)
Following the below formula (Fedo et al., 1995), the correction of CaO* appears to be straightforward. The authors correct CaO for apatite using the P2O5 concentrations and for calcite and dolomite using CO2.
CaO* = mol CaO – mol CO2 (calcite) – 0.5 x mol CO2 (dolomite) – 10/3 x mol P2O5 (apatite)
This, however, causes a common problem with the CIA. Most chemical analyses do not include CO2, and even if it is not necessarily clear how much is hosted in carbonates and how much in organics.
2. McLennan (1993)
McLennan (1993) proposed an empirical correction assuming CaO(silicates) = Na2O if the number of CaO moles after correcting CaO for apatite is greater than that of Na2O.
3. My suggestion
If anhydrite or gypsum is present and S or SO3 concentrations are available (e.g. from X-Ray Fluorescence, XRF), CaO can also be corrected for this: CaO* = CaO – SO3 (anhy./gyps.) (calculate in moles)
Figure 1:
The Al2O3, (CaO + Na2O), K2O (or short A-CN-K) diagram highlighting some weathering trends (see explanations below) and general mineral compositions.
Ternary diagram
If you need a ternary diagram, I wrote an article that explains how to build your own ternary diagram in Excel.
Interpretation of the CIA diagram and values
A word of caution
The CIA was developed for weathering profiles and prediction of weathering trends of igneous rocks. Although the index is often applied to sedimentary rocks, grain-size distributions and sorting can have a strong effect on the CIA. For instance, fine-grained sediments might include a higher proportion of clay minerals than coarser-grained sediments, such as sandstones, affecting e.g., Al2O3 concentrations. In addition, like Garzanti and Resentini (2016) point out, “the mineralogy and consequently the geochemistry of sediments may undergo substantial modifications by diverse physical processes during transport and deposition, including recycling and hydraulic sorting by size, density or shape, and/or by chemical dissolution and precipitation during diagenesis”, requiring caution when interpreting chemical weathering indices.
Garzanti et al. (2013)
There is a good match between theoretical and experimental results and geochemical data from modern weathering profiles when plotted as molecular proportions on the A-CN-K diagram:
- Early weathering stages are characterized by depletion in CaO, Na2O, and K2O (feldspar dissolution) resulting in trends subparallel to the CN-A axis (green and blue arrows in Figure 1).
- Mafic igneous rocks, such as gabbros, contain plagioclase and weathering trends plot close to the CN-A axis towards the smectite composition (green arrow in Figure 1).
- Felsic rocks, such as granites, on the other hand contain K-feldspars. Their weathering trends (still subparallel to the CN-A axis) plot towards illite on the K-A axis (blue arrow in Figure 1)
- Later stages of weathering are characterized by further K2O depletion with data trends plotting along the K-A axis towards kaolinite (and/or gibbsite) (orange arrow in Figure 1).
Some intial CIA values for different igneous rocks:
- Basalt 0 to 45
- Granites and granodiorites 45 to 55
- Idealized muscovite ≈ 75
- Illite, montmorillonite, and badelites ≈ 75 to 85
- Kaolinite, gibbsite, and chlorite close to 100
Some CIA values to estimate the degrees weathering:
- < 50 to 60 initial stages of weathering
- 60 to 80 intermediate degrees
- > 80 to 100 extreme degrees
A final suggestion
Fedo et al. (1995) point out the effect of potassium metasomatism on the CIA values and suggest a K-metasomatism correction. See reference for further details.
References in this Article
Fedo, C.M., Nesbitt, H.W., and Young G.M. (1995): Unraveling the effects of potassium metasomatism in sedimentary rocks and paleosols, with implications for paleoweathering conditions and provenance. Geology, 23/10, 921–924. [Link]
Garzanti, E., Padoan, M., Ando, S., Resentini, A., Vezzoli, G., and Lustrino, M. (2013): Weathering and relative durability of detrital minerals in equatorial climate: Sand petrology and geochemistry in the East African Rift. The Journal of Geology, 121 (6), 547-580. [Link]
Garzanti, E. and Resentini, A. (2016): Provenance control on chemical indices of weathering (Taiwan river sands). Sedimentary Geology, 336, 81-95. [Link]
McLennan, S.M. (1993): Weathering and global denudation. The Journal of Geology, 101, 295-303. [Link]
Nesbitt, H.W., and Young, G.M. (1982): Early Proterozoic climates and plate motions inferred from major element chemistry of lutites. Nature, Vol. 299, 715-717. [Link]
Nesbitt, H.W., and Young, G.M. (1984): Prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations. Geochimica et Cosmochimica Acta, Vol. 48, 1523-1534. [Link]