Geochemistry and Mineralogy - Chemostratigraphy.com

Element – Mineral Associations

Chemostratigraphy is based on element distributions in the geological record, e.g., strata. The element assemblage is determined by the mineralogical composition of the rocks. It is therefore important to understand the element-mineral associations for a chemostratigraphic interpretation of element data.

Major Elements

A common definition of major elements is that they have concentrations of more than 1% by weight. In geochemistry, this is further extended to the elements and minerals that make up the Earth’s crust. However, only eight elements (oxygen, silicon, aluminum, iron, calcium, magnesium, sodium, and potassium) are present in amounts greater than 1%, but these sum up to almost 99% percent (Figure 1).

Figure 1: Average geochemical composition of the Earth’s crust.

The Earth’s crust (comprising of the oceanic and continental crust) is made up of about 95% igneous and metamorphic rocks, overlain by thin layers of sedimentary rocks (ca. 4% shale, 0.75% sandstone, and 0.25% limestone). Thus it is valid to include titanium, manganese, and phosphorus to the major elements, i.e. those that make most of the rock-forming and some other minerals (Figure 2).

Figure 2: Average mineralogical composition of the Earth’s crust.

Trace Elements

Trace elements, on the other hand, commonly have abundancies of less than 0.1 % by weight. Their concentrations are usually expressed in parts per million (ppm) or even parts per billion (ppb); for the latter for instance the elements of the platinum group.

For brevity, we discuss the rare earth elements (REE), i.e. the lanthanides (La to Lu) and actinoids (i.e., Th and U), from the other trace elements. The lanthanide REE can further be grouped into

light and heavy rare earth elements, LREE (La-Sm) and HREE (Eu-Lu) respectively.
OR light, middle (or medium), and heavy rare earth elements, LREE (La-Pm), MREE (Sm-Gd), and HREE (Tb-Lu).

Note 1: Sc is commonly grouped to the LREE, and Y to the HREE, due to their similar chemical behaviors and properties.

Note 2: Some authors exclude Eu from this classification, as it has two valencies, +2 and +3.

Good to know

Percent to ppm

The conversion factor between percent and parts per million (ppm) is 10,000; i.e., 1% = 10,000ppm.

To establish an element–mineral affinity, it is recommended to compare and analyze geochemical data together with mineralogical data, e.g., from XRD or SEM (automated mineralogy). Statistical approaches (e.g., correlation/covariation matrices, cluster analyses, principal component analyses, etc.) or even graphical techniques (binary and/or ternary diagrams), may assist in establishing element-mineral relationships. A complicating factor is, however, that many elements have several mineral affinities. For instance, Si is the main constituent in silicates (7 groups with approximately 600 minerals), Al in alumosilicates, such as Feldspars and clay-minerals, or K and Rb in K-feldspars, mica/muscovite, and minerals of the illite group.

Prefix

Info Box

Many elements have several mineral affinities. Besides O, Si and Al are probably the most extreme examples. Si, for instance, is the main constituent in silicates comprising of 7 groups with approximately 600 minerals. Al in alumosilicates, such as Feldspars and clay-minerals, where the latter comprises of 9 mineral groups.

The most common element – mineral association

This article has been altered from its initial content (Nov. 2021). There used to be a table with elements linking to separate pages for each listing the most common element – mineral association. For structural/technical reasons of chemostratigraphy.com the table has been moved to ‘Element-Mineral Links‘.

The Feldspar Compositional Ternary Diagram

Minerals of the feldspar group consist of three compositional end members. This can be illustrated in the feldspar compositional ternary diagram.

Feldspar minerals are the most abundant constituents of igneous rocks. Despite being sensitive to weathering and alteration, feldspars are abundant (second after quartz) in arenaceous sedimentary rocks, either in form of detrital grains or as secondary, authigenic phases.

Classification of the feldspar group minerals

The generalized chemical composition of feldspars is X(Al,Si)₄O₈, where X is commonly potassium (K), sodium (Na), or calcium (Ca); – rarely X can be barium (Ba), rubidium (Rb), or strontium (Sr).

Feldspar minerals can generally be classified by their chemical composition and expressed in a ternary system of KAlSi₃O₈ – NaAlSi₃O₈ – CaAl₂Si₂O₈ (potassium, sodium, and calcium feldspar, respectively).

KAlSi₃O₈	K-feldspar	Or (orthoclase)
NaAlSi₃O₈	Na-feldspar / Na-plagioclase	Ab (albite)
CaAl₂Si₂O₈	Ca-feldspar / Ca-plagioclase	An (anorthite)

The three compositional end members of the feldspar group.

The differentiation of the feldspar minerals is commonly expressed in reference to their NaAlSi₃O₈ (albite) or Ab mole percentage (see below).

The feldspar compositional ternary diagram

While feldspars between the compositional end-members KAlSi₃O₈ and NaAlSi₃O₈ are referred to as alkali feldspars, the series between NaAlSi₃O₈ and CaAl₂Si₂O₈ are called plagioclase feldspars.

A ternary diagram, based on these three endmembers, can therefore represent the feldspar compositions accordingly (Figure 1).

Feldspar-Ternary-Diagram — Figure 1: The feldspar compositional ternary diagram.

Alkali feldspars

The alkali feldspar compositions and crystallographic symmetries depend on their crystallization temperatures.

Thus, K-feldspars occur in different crystallographic symmetries, i.e., monoclinic and triclinic, depending on their formation temperatures.

Highest temperature:	sanidine	(monoclinic)
Lower temperature:	orthoclase	(monoclinic)
Lowest temperature:	microcline	(triclinic)

The three polymorphs of K-feldspar.

High temperature

At high temperatures (≥ 1000 °C), a solid solution exists between NaAlSi₃O₈ and KAlSi₃O₈. Nevertheless, a change in the symmetry between triclinic anorthoclase (Ab100 to Ab63) and monoclinic sanidine (Ab63 to Ab0) occurs. (Most volcanic rocks are typically high-temperature products.)

Anorthoclase ((Na,K)AlSi₃O₈) occurs in high-temperature sodium-rich volcanic and shallow intrusive igneous rocks. Slow cooling allows the separation of Na- and K-rich feldspars within the same specimen (see below).

Low temperature

At lower crystallization temperatures (≤ 650 °C), however, there is no solid solution. The two feldspars (K- and Na-rich) are therefore separated (miscibility gap; see dashed line in Figure 1).

Interestingly, this separation can result in the intergrowth of two feldspars often in form of laminae. Perthite is the term for K-feldspar that is intergrown by Na-feldspar, while antiperthite is the name for Na-feldspar intergrown by K-feldspar. Mesoperthite is the label for ± equal proportions of K- and Na-feldspar (Le Maitre et al. 2005). (Low-temperature or slow cooling is common for plutonic rocks.)

Plagioclase feldspars

A solid solution exists for the Na- and Ca-rich plagioclase feldspars (albite and anorthite respectively). The plagioclase series is differentiated into six compositional ranges (Figure 1), which is expressed through their Ab mole percentage:

Albite	(Ab100-90)
Oligoclase	(Ab90-70)
Andesite	(Ab70-50)
Labradorite	(Ab50-30)
Bytownite	(Ab30-10)
Anorthite	(Ab10-0)

All members of the plagioclase series, however, crystallize in the triclinic symmetry.

Hint:

Build your own ternary diagram

I used Excel to generate the compositional ternary diagram for feldspars. Would you like to build your own ternary diagram for feldspars or any other application? You can follow the instructions in the article “How to plot a Ternary Diagram in Excel”.

References

Le Maitre, R.W.; Streckeisen, A.; Zanettin, B.; Le Bas, M.J.; Bonin, B.; and Bateman, P. (2002). Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks (2 ed.). Cambridge University Press. p. 20. [Link]

Chemical Index of Alteration (Nesbitt & Young, 1982)

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 CO_{2 (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 – SO_{3 (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:

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]