Pseudo-Icosahedral Pyrite in Colombian Emerald
The author recently examined a 2.88 ct emerald containing a beautiful metallic yellow pseudo-icosahedral pyrite crystal with truncated edges (see above). The emerald was identified as Colombian based on its jagged three-phase inclusions and trace element analysis using X-ray fluorescence. The included crystal’s color, luster, and shape were characteristic of pyrite, which is sometimes found in emeralds from various countries (S. Saeseaw et al., “Geographic origin determination of emerald,” Winter 2019 G&G, pp. 614–646). Pyrite, also known as “fool’s gold,” belongs to the isometric crystal system and has an ideal chemical composition of FeS2.
Like other isometric or cubic minerals, including diamonds and garnets, pyrites exhibit various morphologies depending on growth parameters such as temperature and/or degree of supersaturation (J.B. Murowchick and H.L. Barnes, “Effects of temperature and supersaturation on pyrite morphology,” American Mineralogist, Vol. 72, 1987, pp. 1241–1250). A regular icosahedron is one of the five classic Platonic solids, but it cannot exist in a crystal because it contradicts Haüy’s law of rational indices, which states that the intercepts of any crystal face along the crystallographic axes are either equal to the lattice constant lengths or some simple whole number multiples of them. Goniometric studies by Romé de L’Isle from 1783 showed that a pseudo-icosahedral form, often found in pyrite, is a combination of a regular octahedron and an irregular pentagonal dodecahedron known as a pyritohedron.
This pyrite crystal in Colombian emerald would be a welcome sight for any gemologist.
