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Chemical-Index-of-Alteration

Chemical Index of Alteration (Nesbitt & Young, 1982)

Christian Scheibe

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)

Chemical Index of Alteration diagram
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:

  1. 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)
  2. 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]

10 thoughts on Chemical Index of Alteration (Nesbitt & Young, 1982)

    1. The correction with P2O5 is for apatite (Fedo. et al., 1995). CaO concentrations are usually much higher than P2O5, unless you have phosphate minerals other than apatite.
      Assuming you did the mol conversion correctly,
      a) Do you have carbonates and/or anhydrite/gypsum in your samples? Do you have CO2 values for corrections?
      b) What is your like phosphate mineral, apatite or other (e.g. monazite)?
      c) What is the source of your data (analytical instrument) and do you have confidence in the data quality?

      Reply

  2. Dear Christian,
    I am wondering if we could calculate and apply CIA by using microanalyses obtaning from EMPA in mudstones.

    Thank you in advance,
    Vayia

    Reply

    1. Dear Vayia.

      That is an interesting question. I would say, it depends on the number of measurements/data points. Is the average composition from all your measurements statistically representative for the whole-rock composition? You may think of point counting with a (petrography) microscope, but on a much smaller scale.
      A mudstone/mudrock comprises clay minerals and silt-size minerals, e.g. quartz, feldspars, micas, and accessory minerals. Thus all these need to be taken into account in the correct proportions.
      Does that make sense?

      Another thing to consider is where does your mudstone come from and where has it been deposited? Transport from source to sink will lead to sorting and possible physical weathering. The latter will break down labile mineral grains and make them more prone to chemical weathering. Both sorting and/or a combination of physical and chemical weathering will affect the CIA values.
      A mudock has commonly a high percentage of clay minerals and the CIA will thus be high.
      There is an interesting book chapter by Bahlburg, H. and Dobrzinski, N. (2019), ‘A review of the Chemical Index of Alteration (CIA) and its application to the study of Neoproterozoic glacial deposits and climate transitions’ (free download, just google it), which gives a good overview on the above mentioned processes.

      I hope this helps a bit.
      Please let me know how it goes.
      Best wishes, Christian

      Reply

  3. Hi Dr. Christian,
    – Regarding the calculation of CO2, is there any of the rock-eval parameters that can help to differentiate the inorganic carbon (MINC or S4CO2 for example)?

    – In your suggestion for the calculation of CaO*, is the formula in molecular proportions as well or weight %? and Shall we use this formula only if we have gypsum/anhydrite or just if we have CaO and SO3 concentrations we can calculate CaO*?

    Reply

    1. Dear Mutasim,
      Thank you for reaching out with your interesting questions.
      CaO* refers to the CaO (in molecular proportions) of the silicate fraction only, i.e. here after weathering/alteration. Thus, mainly feldspars, clay minerals and other Ca-bearing silicates. Carbonates, anhydrite/gypsum, and other diagenetic added minerals need to be excluded.

      – I am not too familiar with pyrolysis, so here as I understanding it: Rock-Eval instrument can give you MinC, i.e. the mineral carbon (C) in wt% (I assume that are all carbonate minerals present, e.g. calcite, dolomite, siderite, etc.). You may need to convert it to CO2 depending on the method you use you calculations for CaO*. However, for the CIA formula CaO* has to be in molecular proportions. As I understand, S4CO2 is the residual Carbon after completion of pyrolysis, and is (together with S4CO, S3, S2, and S1) used to compute the sum of TOC. Another instrument, which I am aware of, reports CC (carbonate carbon in wt%) from S5CO2. I assume that is probably the same as MinC.

      – Yes, the correction for anhydrite/gypsum is in moles, too. (I will make a note in the article, thanks for pointing it out.) Obviously there is another method using wt% (anhydrite: 41.19% CaO, 58.81% SO3; gypsum: 32.57% CaO, 46.50% SO3, 20.93% H2O). SO3, however, does not mean automatically that you have anhydrite/gypsum; it can come from other sulphates such as barite. Depending on the instrument, i.e. if it is capable of measuring S, SO, SO3, etc. you can assign those to different mineral groups. XRF will report sulphur either as S or converted to SO3. Thus S/SO3 from XRF can come also from sulphides, such as pyrite.

      Many people neglect the correction, e.g. when there are no or low quantities of carbonates (and I believe some even do not converted to moles).
      The CIA is a complicated index when you want to do it scientifically correct and have only incomplete data (which is usually the case). Pyrolysis data and good mineralogy data, e.g. XRD, would be definitely an advantage.

      Reply

      1. Thank you very much, Dr. Christian, for the answers and the information provided.
        Here are the CO2- and SO2-related parameters and their definitions that we can get from Rock Eval-7:
        ❖ S3 = the amount of CO2 (in milligrams CO2 per gram of rock) produced during pyrolysis of kerogen.
        ❖ S3CO2 (mg/g rock): The CO2 from organic sources released during the pyrolysis phase.
        ❖ S3’CO2 (mg/g rock): The CO2 from mineral sources released during the pyrolysis phase.
        ❖ SO2 pyrolysis (mg/g rock): The SO2 measured during the pyrolysis phase.
        ❖ S4CO (mg/g rock): The CO from organic sources released during the oxidation phase.
        ❖ S4CO2 (mg/g rock): The CO2 from organic sources released during the oxidation phase.
        ❖ S5 (mg/g rock): The CO2 from mineral sources released during the oxidation phase.
        ❖ SO2 oxidation (mg/g rock): The SO2 measured during the oxidation phase.
        ❖ MinC = the amount of mineral carbon content (weight %). This includes pyrolysis, mineral carbon, and oxidation mineral carbon.
        ❖ Total S= Total Sulfur content in weight %.

        Reply

  4. Can I use CIA in carbonate rocks?? I trying to be calculated CIA in limestone and dolomite but the values in dolomite was very low (very less than 50)

    Reply

    1. Dear Farah,
      The CIA (and other alteration indices, such as PIA, WIP, etc.) are for non-carbonate rocks, i.e silicate minerals only. For instance, the CIA includes the CaO* component, which is a corrected CaO value for the silicate fraction only. If not corrected, the high CaO value goes into the denominator of the formula and will strongly lower the results. I would strongly recommend not to use these indices for carbonates. If you should come along an index for carbonates, please let me know about it.
      However, keep in mind: a) where do the other elements (Al2O3, K2O, etc.) come from, e.g. continental dust; and b) CIA is a chemical alteration index particularly use for weathering profiles. Weathering of carbonates may encompass karstification and thus secondary input of (weathered) material.
      I hope that helps.

      Reply

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