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.
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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‘.