Growing Crystals with Hourglass Inclusions Under the Microscope 
by James Benko, Zeeland, MI USA
Looking at perfectly formed crystals under the microscope is always fascinating. Ordinary "chemicals" around the home such as salt and sugar can be crystallized from water solutions on a microscope slide to provide interesting specimens. Even with random crystallization's that will produce mostly distorted specimens, some perfectly formed cubes and rectangles from table salt and orthorhombic crystals from sucrose (sugar) make interesting specimens for viewing. This study can be continued on other materials from the kitchen and medicine cabinetcitric acid from unsweetened Kool-Aid, aspirin, starch grains, etc.to provide a number of interesting crystals to view under the microscope with both regular and polarized light. Although these make interesting specimens for viewing, they are still "ordinary" crystal specimens.
Occasionally, however, some unusual crystallization phenomenon is discovered, studied, and described in the literature that piques the curiosity of chemists, mineralogists and crystallographers. One such phenomenon is the growth of hourglass inclusions in the crystal lattice. Hourglass inclusions were originally observed for various chemical pairs over 150 years ago and were extensively studied in the 1930s by Buckley in Manchester, England. Out of over 16,000 combinations of chemical pairs he studied, he was able to document only a handful of materials that crystallize in this manner. As one might expect, such unusual crystal growth is quite rare in crystallography. 

In the following example, acid fuchsine dye will grow under favorable conditions in the crystal lattice of potassium sulfate, not turning the whole crystal red, as one might expect, but forming a red color only in certain regions of the crystal lattice to form an hourglass looking structure. (For a more thorough discussion of the crystallography taking place see reference (1) below.) This is perhaps one of the easiest to demonstrate and study hourglass crystal structures. 

Directions for preparing micro specimens of these crystals have been taken from the original reference (1) and adapted for growth on a microscope slide (2). These crystals are very interesting and unusual specimens to observe under the microscope in both regular and polarized light.

Procedure:

  1. Prepare a 10% solution of potassium sulfate by dissolving 10 grams of K2SO4 in 90 ml of deionized water. Since this is at, or near saturation, the solution may have to be heated slightly to get all of the solids dissolved.

  2. Prepare an 0.1% solution of acid fuchsine dye by dissolving approximately 0.1 gram of acid fuchsine in 100 ml of deionized water. (Acid Fuchsin, sodium salt, Cat. No. 33,270-4, Aldrich Chemical Co., Milwaukee, WI, 53233, USA) 

  3. Add 10 drops of 10% potassium sulfate solution to a microscope slide. Then add 1 drop of 0.1% acid fuchsine dye solution and mix well. 

  4. Examine the prep after about an hour to observe hourglass crystals growing in the mixture. 

  5. Repeat Steps 3 and 4 if hourglass crystals are not observed. The crystals form only within certain concentration ranges of these materials and sometimes the drop sizes are not sufficient to provide these conditions.

  6. Isolate and dry individual crystals for study under the microscope. These can be permanently stored in dry mount preps (3). 
For those microscopists who do not have access to the chemicals mentioned above but would still like to observe these crystals firsthand, you can contact me for a slide prep of some of these crystals. I will trade you for an interesting slide prep of your own or send you a slide for a small fee to cover the cost of a slide mailer and postage. 

You may contact me by e-mail Comments to the author sent via our contacts page quoting page url plus : ('jbenko','')">Jim Benko.

References:-

1. Bart Kahr, Jason Chow, and Matthew Peterson, Organic Hourglass InclusionsA Review of Past and Recent Work and a Student Experiment, Journal of Chemical Education, Volume 71, Number 7, July 1994, pp. 584-586. 

2. James Benko, Letters to the Editor, Organic Hourglass Inclusions, Journal of Chemical Education, Volume 72, Number 10, October 1995, p. 956. 

3. James Benko, Micscape Practical Tip: A Quick and Easy Way to Make Drymounts, Micscape Magazine, July, 2000

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