GARNET in SCHIST
This is a photo of a small (~2mm) garnet (possibly almandine) found in a specimen of mica schist from the Black Hills.
Approximate Photo Location (Side 1)
Magnification: ~4.3X (crop)
Field of view: ~1/4” x 5/16“ (5.6mm x 8.4mm)
Images in focus stack: 70
The name garnet is derived from the Latin word “granatium” or “granatus,” which means pomegranate, referring to the similarity between the crystals and pomegranate seeds (1). Garnets are found in a variety of colors, including red, yellow, and green (4). They are used to produce gemstones and abrasives, in water filtration systems, and for waterjet cutting and sand blasting (1, 4). (The hardness of garnets is in the 1,300 range on the Knoop scale, where Copper is ~150, quartz ~800, and diamond ~8,000)(5, 6). Almandine is the most common type of garnet (frequently in reddish/brownish colors) and is widely used in jewelry (2, 3).
In terms of crystallization, garnets commonly form as rhombic dodecahedrons, which are 12-sided structures (1). The shape is overlaid on the photo below, and accompanied by an illustration. Interestingly, domestic bees sometimes build their honeycombs in this form. The front of the honeycomb is open and shaped like a hexagon. However, the far (back) end is enclosed with three diamond shapes (rhombuses) to form the dodecahedron. Triangles, squares, and hexagons are the shapes (regular polygons) that can be tiled together on a plane without gaps (7). If the perimeters of these are held constant, hexagons enclose the greatest area (7). Similarly (holding the volume constant), an end cap made with rhombuses takes up less surface area than one with a flat base (8, 9). In other words, by choosing this shape a bee has to convert less honey to produce the wax for the honeycomb walls, leaving more honey for the bees to consume and survive (7, 8). So how does this relate to garnet crystal formation? I have no idea :). There does appear to be a relationship between shape, chemical composition, and the rate of temperature change during formation (with this form being more common where the temperature change is slower)(10).
1. Feick, K. (n.d.). Garnet. Retrieved from the University of Waterloo Earth Sciences Museum.
2. Anthony, J.W., Bideaux, R. A., Bladh, K. W., & Nichols, M.C. (Eds.). (2001). Almandine. In The Handbook of Mineralogy. Retrieved from the Mineralogical Society of America.
3. University of Wisconsin-Madison. Almandine. (n.d.). Retrieved from the Virtual Museum of Molecules and Minerals.
4. King, H. M. (n.d.). Garnet. Retrieved from Geology.com.
5. Ted Pella, Inc. (n.d.). Hardness tables. Retrieved from Ted Pella.
6. Glaeser, W. (1992). Chapter 1: Steels (Table 1.7). In Materials for Tribology. Retrieved from Google Books.
7. Bird, A. (2015). Apiological: Mathematical speculations about bees (Part 1: Honeycomb geometry). Retrieved from The Aperiodical.
8. Cardil, R. (n.d.). A property of minima in honeycombs. Retrieved from Matematicas Visuales.
9. University of Pittsburgh. (2001). Cannonballs and honeycomb: Kelvin. Pitt MathZine, Fall 2001. Retrieved from the Department of Mathematics.
10. Sepahi, A. A. (2007). A detailed study of morphology and chemistry of garnet crystals with suggestion of new subdivisions: Data from pelitic schists, hornfelses, and aplites of Hamedan Region, Iran. In the Iranian Journal of Science & Technology, 31(A3). Retrieved from ResearchGate.
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