This exercise is designed for a mid/upper-level undergraduate geology course on the principles of mineralogy.

Students should have knowledge of basic chemistry and of minerals equivalent to what they would learn in an introductory geology class.

This activity is the 16th of 36 mineralogy exercises and is used around the middle of the course. This activity is a stand-alone exercise, but is part of a larger volume of classroom and laboratory activities from "Teaching Mineralogy," a workbook published by the Mineralogical Society of America, Brady, J., Mogk, D. W., and Perkins, D., (editors), 1997,406 pp.

• Be able to explain what a solvus is and to interpret solvus diagrams.
• Learn to use X-ray data to determine whether the alkali halide system is a good analog for feldspars.

• Understand the nature of experimental studies.
• Learn to deal with ambiguous experimental results.
• Learn to combine information of different sorts to reach conclusions/interpretations.
• Critically analyze results and separate what is valid from what is not.

• Improve experimental technique.
• Know the difference between accuracy and precision.

This complex experimental investigation uses alkali halides (NaCl, KCl, and mixtures of both) to simulate the melting of alkali feldspars which melt at too high of temperatures to work with in lab.

Three hypotheses are tested:

1. It is possible to crystallize alkali-chloride salts from a magma with any composition between NaCl and KCl.
2. Because K+ and Na+ do not have the same ionic size, the atomic spacing in alkali chlorides will vary systematically with composition.
3. Alkali chlorides are equally stable at high (just below liquidus) and low (subsolidus)temperatures.

This project takes more than one class period, depending on how many students are in the class, because there will be lines at the scales, oven, and XRD. It is advisable to introduce the lab in class and have students complete various parts on their own time. There are three main parts.

• Part one: Synthesize all alkali halide compositions at high temperature (hopefully above the solvus.
• Part two: Put grown crystals back in an oven at lower temperature to see if they will unmix.
• Part three: Write a report evaluating and interpreting all results, relevant graphs, and the above three hypothesis.

• Mineralogy Course Summary and Goals
• Mineralogy Course Syllabus
• Brady, Mogk and Perkins, 1997, Teaching Mineralogy, Mineralogical Society of America, pp 91-96
• Barrett, W.T., and Wallace, W.E., 1954, Studies of NaCl-KCl solid solutions, J Am. Chem. Soc. 76, 366-69
• Levin, E.M., Robbins, C.R., and McMurdie, H.F., 1964, Phase Diagrams for Ceramists, Am. Ceramic Soc., Columbus, 601 p