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Synthetic Alkali Halides

Dexter Perkins
,
University of North Dakota
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This activity was peer reviewed prior to publication in the Teaching Mineralogy Workbook.

This teaching activity was originally published in: Brady, J., Mogk, D. W., and Perkins, D., (editors), 1997, "Teaching Mineralogy," a workbook published by the Mineralogical Society of America, 406 pp. All teaching activities in this volume received two external peer reviews from mineralogy faculty focused on content and pedagogy, and a final review by the co-editors to comply with the publication standards of the Mineralogical Society of America.



This page first made public: Aug 7, 2006

Summary

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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Context

Audience

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.

Goals

Content/concepts goals for this activity

Higher order thinking skills goals for this activity

Other skills goals for this activity

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.

Determining whether students have met the goals

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