The ternary diagram geology uses to show the relationships between similar minerals come up a lot in my silicates articles. Ternary diagrams can describe ANY relationship with three related components. It looks like a triangle with different chemical formulas of rocks at the corners. Understanding how to read the ternary diagrams is like reading a simple language that will help you get familiar with the dynamic compositions of natural rocks.
In nature, large pieces of pure minerals are pretty rare. Rather, we see composite rocks with percentages of different, similar minerals. For example, peridotite has a large percentage of olivine (40%+), along with varying amounts of amphibole, biotite, and other minerals.
How to Read a Ternary Diagram
Ternary diagrams look complicated, but learn how to read a ternary diagram and you will see how useful they are.
Each side of the equilateral triangle is a gradient from zero to one hundred percent. A particular rock holds a “coordinate,” a percent from each side of the triangle, that adds the three sides to one hundred percent.
To make this coordinate system work, the percent gradients along the sides are measured from an altitude of the triangle. An altitude is a line perpendicular to one side and extends to the opposite side. Let the picture explain.
The line that’s parallel to (the line connecting) BC tells you percent A.
The line that’s parallel to AC tells you percent B.
The line that’s parallel to AB tells you percent C.
The percents always add up to 100%.
Corners and Side points in ternary plots
Yellow: 100% component A
Blue: 100% B
Purple: 100% C
For yellow: A line parallel to the bottom passing through the star shows A is 100%, a line parallel to the right side shows B is 0%, a line parallel to the left side shows C is 0%.
Yellow: 50% A, 0% B, 50% C
Blue: 50% A, 50% B, 0% C
Purple: 0% A, 50% B, 50% C
For blue: A line parallel to the bottom passing through the star shows A is 50%, a line parallel to the right side shows B is 50%, a line parallel to the left side shows C is 0%.
Try reading ternary plot examples
At this link you can look at any specified point on a ternary plot. Ternary plots are used to graph any three variable relationship actually, as in soil compositions and epidemiology. Some people have even adapted ternary diagram to baking.
For example, the blue line is opposite A so it reads on the right side of the triangle.
The purple line is opposite B so it reads on the left side of the triangle.
The yellow line is opposite C so it reads on the bottom of the triangle.
So that is how to read the three constituent compositions of a point on a ternary plot. In mineral and petrology ternary plots, different composite substances are defined in regions on the diagram.
Ternary Plots in Geology Composite Rocks Examples (Super informative!)
In geology, the three corner components are compounds (such as SiO2, Al2O3) and since most rocks have upwards of seven different compounds, the plots are often simplified to show the most important information. The example plots in this section are from Prof. Stephen A. Nelson from Tulane University and Eskola’s important 1920 publication.
An example of a pure rock would be a sandstone that is mainly quartz (SiO2), quartzite. Limestone can also be almost entirely calcite (CaO and CO2). Most rocks, like shales, basalts, granites, and dolomites have a few different main components. (Read about the main different types of rocks here for all the background!)
There are two main regimes for ternary plots in geology that describe the majority of rock types. These are called the ACF and AKF plots for short.
ACF Ternary Geology plots Examples
In this plot for metamorphic rocks, A = Al2O3, C = CaO, and F = FeO + MgO. Ternary plots in geology often combine elements together like this to streamline the information. Fe and Mg can swap out for each other, so the proportion of the sum is regular.
There is a little more complexity in order to achieve simplicity. The percent composition A is really the amount of Al2O3 after removing contributions from Na2O and K2O, and C is the amount of CaO after removing apatite and quartz.
We also ignore small contributions, like trace elements.
For example, I want to know how hornblende differs from chlorite. I can see chlorite on the far right edge, meaning it contains almost none of component C, CaO. Hornblende contains more calcium oxide than chlorite (or biotite) and more of the A and F contributions. Another example, muscovite on the top corner contains nearly all Al2O3, after taking out the SiO2 (quartz) part. (Hornblende, chlorite, and muscovite are all silicate minerals.)
Some of those same silicate minerals can be compared differently on an AKF diagram.
AKF Ternary Plots in Geology Examples
In AFK ternary plots, A = Al2O3 + Fe2O3 – Na2O – K2O – CaO (aluminum oxide and iron oxide with sodium, potassium, and calcium oxides ignored), K = K2O, and F = FeO + MgO + MnO.
Compositional phase diagram of the different minerals that constitute the
Ternary plots for composite rocks are often most valid only for a particular temperature and pressure. So just note that if we go further under Earth or have different ambient conditions, the lines separating different minerals shift and chemical bonds break and reform.
ACF and AKF diagrams can be combined and projected in 3D to reveal even more relationships. Many rock groups, such as Plutonic rocks that form way underground, are also even better understood by a similar, four pointed graph called a QAPF diagram on a DOUBLE triangle plot.
Ternary plots both neat and Useful
With so many rock types and nearly infinite compounds possible, ternary diagrams help organize a little of the geology the best we are able. This way, researchers can better form hypotheses to test, and we can understand shared properties between similar rocks. When you practice how to read a ternary diagram, it’s a bit of a fun brain teaser that gives you a deeper understanding of the trends.
Sources
Eskola, Pentti Eelis. “The mineral facies of rocks.” (1920): 143-194.
