Mineral Chemistry - Chemostratigraphy.com

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‘.

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]

Category: Mineral Chemistry

Element – Mineral Associations