Mineral Structural Formulae & Stochiometry

Tuesday 1:30pm-2:40pm Burge Union Forum C/D
Share-a-Thon Part of Share-a-Thon Presentations


Brendan Hanger, Colorado School of Mines
Greg Yaxley, Australian National University


I will present a brief outline of the activity and show how the basics of how it works. I will also be able to offer a detailed run-through if others are interested.


This is an Excel-based activity used in mineralogy and petrology courses to introduce the concept to determining the mineral structure formulae (or cations per atomic functional unit) from given mineral oxide data, as obtained using an electron microprobe. This activity serves four main purposes: 1) introducing students to geochemical data and its format (weight % oxide); 2) understanding the new to perform checks for data quality; 3: deepening understanding of mineral solid solutions; and 4) learning to use Excel effectively when dealing with repeated calculations. The first time I use this activity, I have students develop an excel spreadsheet that will convert oxide weight % data into cation data appropriate for reading off the exact structural formula when the type of mineral is known (e.g. Mg1.81Fe0.17Ni0.02SiO4 for an olivine). I repeat the activity throughout the semester and increase the complexity of the minerals, eventually presenting data of unknown minerals, requiring students to determine the mineral in addition to the formula (e.g. is it olivine, clinopyroxene, feldspar), as well calculating end-members and Mg-numbers. Advantages of this approach include students learning to develop a functional spreadsheet, and the eventual ability to interpret whether an analysis is of sufficient quality to be used.


The target audience for this activity are my sophomore mineralogy and igneous/metamorphic petrology courses. Over the course of the two semesters I use variations on the activity multiple times, initially varying the mineral examples used, and later presenting data from rock suites, often with the mineral names missing. At various times I have used it as in-class activity with minimal assessment, in assessed lab classes or as part of assessed problem sets.

Why It Works

I find this activity particularly effective as it serves multiple goals, and is easily altered to fit different parts of the courses. As well as building a strong understanding of mineral chemistry and solid solutions, it also develops students' abilities to construct spreadsheets in a way that can minimize future work. I particularly like the idea of having students construct the spreadsheet, as many similar activities I have seen operate on a more black=box model where data goes in and answers come out, not requiring students to understand the process.

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