Students learn how to read coordinates on such a graph and to describe points, lines, and areas. Part I introduces the concept of a triangle or ternary diagram for plotting data described by three coordinates. The exercise is divided into three parts. 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. These characteristics make triangle diagrams convenient for visualizing compositional data. Triangle diagrams allow plotting three coordinates on a planar graph if the values add up to 100%. Another way is to plot their compositions using a triangle diagram. One way to recognize their similarity is to determine their physical properties and study their chemical formulas. Yet even within this closely related group, separate minerals may look very different. All of them are silicate minerals, and together they make up more than half of Earth's crust. Geologists use the term 'feldspar' to refer either to a specific mineral or to a group of minerals with similar compositions and atomic structures.
