Initial Publication Date: August 11, 2023

Ternary Diagrams - Practice Problems
Solving earth science problems with ternary diagrams

Plotting data on ternary diagrams

Mineralogy - Plotting mineral compositions

Many mineral groups can be plotted on ternary diagram with respect to three main components to differentiate between different mineral species.

Problem 1: A pyroxene mineral has the following geochemical oxide weight percents (wt.%) seen in the figure below. Plot the pyroxene with respect to the concentrations of CaO to MgO to FeO. When plotted, the data will plot within a labelled field which will allow for the determination of the pyroxene species name. Determine the pyroxene mineral name.


Sedimentology - Sediment classification

Sedimentologists have their own naming scheme based on grain size for sediments. These classifications are most often applied to offshore sediments or sediments with a minimal organic component.

Problem 2: You are given a sample from a core taken offshore in the Gulf of Mexico and asked to give the sample a name based on the Shepherd Classification (see ternary below). After sieving, the data you have gathered on grain size is in the table below. What is the name of this sample based on the Shepherd Classification?


Reading ternary diagrams

Petrology - Extracting and interpreting plotted rock compositions


Problem 3. The ternary diagram below shows three plutonic igneous rocks plotted in the standard International Union of Geological Sciences (IUGS) ternary diagram. Rocks are plotted with respect to the abundance of quartz (Q) - alkali feldspar (AF) - plagioclase (P). These rocks have been collected from a field area that shows several different magmatic pulses making a composite intrusion. The relative rock ages are as follows: red circle - oldest; yellow square - middle; and blue triangle - youngest). Interpret the data plotted on this diagram.


Geomorphology - Dune types


There are many variables that can influence the shape of sand dunes but the ones most often cited by geomorphologists include vegetative cover, sand supply, and wind persistence.

Problem 4: The dunes shown in the left portion of the figure below are located in Oregon Dunes National Recreation Area. Most of the dunes are classified as parabolic dunes, particularly near the coastal forest, and that is reflected by the location of the dot on the dune morphology diagram below.

If the coastal forests are logged, what type of dune will likely establish?

Environmental Geochemistry - Water quality 


Geochemists often look at what is dissolved in water to understand if the water is safe for human use and how water chemistry changes spatially and through time. Major ion composition (cations & anions) can be plotted on ternary diagrams to understand differences in major chemical composition.


Problem 5: The watersheds in the ice-free areas of western Greenland get their water from either ice melt (proglacial) or precipitation (nonglacial). As the ice melts, more nonglacial watersheds form. Proglacial and nonglacial watersheds have very different chemical composition as shown in the anion ternary below.

a) What is the dominant anion (Cl-, SO42-, or HCO3-) in proglacial and nonglacial watersheds?

b) What is the range of HCO3- relative abundances in the proglacial watersheds?


Petroleum Geology - Shale reservoir quality & lithofacies classification


Petroleum geologists need to be able to understand whether a subsurface formation will produce enough oil to be worth the cost of implementation. They use lithofacies classification and carbon content of core samples to understand where productive reservoirs are located. Ternaries dealing with shale and mudstone lithofacies classification compare the abundance of siliceous primary minerals (quartz and/or feldspar, mica), clay minerals, and carbonate.

Problem 6: The data on the ternary diagram below are from Hou et al. (2023) and depict the mineralogical composition of a series of mudrocks from China. The color designation of each of the points is total organic carbon composition of the sample (related to oil content) and allows a fourth variable to be visualized as well. Answer the questions below with respect to this ternary diagram.

a) Which end-member are the majority of samples composed of?

b) What two mudrock classification names do the majority of samples fit?


Next steps

If you feel comfortable with this topic, you can go on to the assessment.

Or you can go back to the Ternary Diagrams explanation page.