Synthetic Alkali Halides

Dexter Perkins
University of North Dakota
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This is a complex experimental investigation of the melting of alkali halides. This project takes more than one class and involves using an X-ray diffractometer.

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

Skills and concepts that students must have mastered

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

How the activity is situated in the course

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.


Content/concepts goals for this activity

  • 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.

Higher order thinking skills goals for this activity

  • 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.

Other skills goals for this activity

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

Description of the activity/assignment

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.

Determining whether students have met the goals

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