Feldspar Minerals and Triangle Diagrams
Skills and concepts that students must have mastered
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Content/concepts goals for this activity
Higher order thinking skills goals for this activity
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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
Download teaching materials and tips
- Activity Description/Assignment: Student Instructions for Feldspar Minerals Activity (Microsoft Word 2007 (.docx) 15.5MB Jun16 19)
- Instructors Notes: Lecture Slides for Feldspar Minerals Activity (Acrobat (PDF) 7.4MB Jun16 19)
- Student Handout for Feldspar Minerals Activity (Acrobat (PDF) 17.4MB Jun16 19)
- Workbook for Feldspar Minerals Activity (Excel 2007 (.xlsx) 45kB Jun16 19)
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