Feldspar Minerals and Triangle Diagrams

Eileen Herrstrom
University of Illinois at Urbana-Champaign
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Initial Publication Date: June 17, 2019 | Reviewed: November 25, 2019


This activity takes place in a laboratory setting and requires ~1.5-2 hours to complete. Students learn how to read a triangle or ternary diagram. They determine physical properties of feldspars and interpret two different feldspar triangles.

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Undergraduate class on introductory physical geology or quantitative reasoning for non-majors

Skills and concepts that students must have mastered

Must know how to identify minerals and and have basic knowledge about using Microsoft Excel (enter data, fill down)

How the activity is situated in the course

This is a laboratory activity that follows lectures on minerals and is the fifth laboratory exercise of the course.


Content/concepts goals for this activity

Read coordinates on a triangle diagram and graph points, lines, and areas

Higher order thinking skills goals for this activity

Collect information about the physical properties of alkali and plagioclase feldspars from samples or images, classify the feldspars based on properties, derive chemical formulas, and illustrate mineral compositions on a triangle diagram

Other skills goals for this activity

Apply the principles of triangle diagrams to barium-bearing feldspars, explain charge balance for complex mineral formulas, and organize compositional data into five groups

Description of the activity/assignment

Geologists use the term "feldspar" to refer either to a specific mineral or to a group of minerals with similar compositions and atomic structures. All of them are silicate minerals, and together they make up more than half of Earth's crust. Yet even within this closely related group, separate minerals may look very different. One way to recognize their similarity is to determine their physical properties and study their chemical formulas. Another way is to plot their compositions using a triangle diagram. Triangle diagrams allow plotting three coordinates on a planar graph if the values add up to 100%. These characteristics make triangle diagrams convenient for visualizing compositional data.

Student materials for this exercise include a Microsoft Excel spreadsheet with with data for alkali, plagioclase, and barium-bearing feldspars and an image file illustrating physical properties (PDF), as well as the instruction sheet. The exercise is divided into three parts.

Part I introduces the concept of a triangle or ternary diagram for plotting data described by three coordinates. Students learn how to read coordinates on such a graph and to describe points, lines, and areas.

In Part II, students work with either actual specimens provided by the instructor or images in the PDF to determine certain properties (color, hardness, cleavage, and striations) of three feldspars (orthoclase, albite, and anorthite). They name the samples and then follow an example to deduce the chemical formulas of these minerals. Finally, they apply knowledge from Part I to develop the classic K-Na-Ca feldspar triangle.

Part III involves working with barium-bearing feldspars including albite, orthoclase, hyalophane, and the pure Ba end member celsian. Students first verify the chemical formulas for the two new feldspars using the concept of charge balance. Then, they interpret a K-Na-Ba triangle diagram by relating compositional data to the four feldspars.

Determining whether students have met the goals

In both the traditional face-to-face and online versions of the course, this activity is assessed based on the answers to the questions. It is also possible to have students submit their completed spreadsheets, although this option works best in a small class.

More information about assessment tools and techniques.

Teaching materials and tips

Other Materials

Supporting references/URLs

Fichter, L.S., 2000, Reading a ternary diagram: Online resource – Accessed 16 June 2019 http://csmgeo.csm.jmu.edu/geollab/Fichter/SedRx/readternary.html

Holland, T., 2004, Triquik.xls: Online resource – Accessed 16 June 2019

King, H.M., 2005, Feldspar: Online resource – Accessed 16 June 2019

Henry, D.J., C.N. Will, P.A. Mueller, 2015, Ba-rich K-feldspar from mafic xenoliths within Mesoarchean granitic rocks, Beartooth Mountains, Montana, USA: Indicators for barium metasomatism: The Canadian Mineralogist, vol. 53, pp. 185-198. doi: 10.3749/canmin.1500026 – Accessed 16 June 2019