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January-February 2013

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Letters

We are writing to inform your readers that the term zultanite, the commercial trade-name for transparent gem-quality diaspore that displays an alexandrite-like color change dependent upon the nature of the incident illumination, has been abandoned in favor of the more accurate mineralogical term gem diaspore. The gem rough comes from a unique deposit in Turkey that also produces diaspore with a chatoyancy that can show a fine cat's-eye effect. A. Murat Akgun, owner of Milenyum Mining Company, Milas Bauxite Establishment (Milas-Mugla, Turkey), had originally coined the commercial name zultanite in 2005 to honor the thirty-six sultans who ruled the Ottoman Empire between 1299 and 1923 prior to the formation of the modern Republic of Turkey. Mr. Akgun has now changed the commercial name of this material simply to gem diaspore.

Editor's note: Readers will recall that Murat Hatipoglu and Steven Chamberlain coauthored the article titled “A Gem Diaspore Occurrence near Pinarcik, Mugla, Turkey” that appeared in the May/June 2011 issue of Rocks & Minerals (pages 242–249, with a cover photo tie-in).


John White's first Mineral Mystery column (March/April 2012, pages 181–183) concerned altered pyrite with partial rims of quartz surrounding the crystals, though not on all sides. These are found in phyllite, a fine-grained metamorphic rock between slate and schist in degree of metamorphism. Unaltered pyrite samples do not contain the quartz rims. John suggested that the alteration of the pyrite produced sulfuric acid and heat and that these led to the precipitation of the quartz.

I would like to suggest another, quite different explanation. In 1986, Steve Chamberlain and I collected a large number of pyrite cubes embedded in phyllite from a road construction site near Sherbrooke, Quebec. These pyrite crystals also have rims of quartz, white and fibrous. The rims are thick in some areas and thin or absent in others. All rims are found in the same position in a given specimen, regardless of the orientation of the pyrite crystals. That is to say, the location of the rims seems to be a feature of the rock and not of the pyrite crystals.

What makes these different from John's specimens is that the pyrite is quite fresh and unaltered! After some research, I learned that these rims are known to metamorphic petrologists as pressure shadows and that they are often seen in various metamorphic rocks associated with impervious objects that are larger than the grain of the rock. A web search will turn up a number of examples.

Adolph Pabst published a study of this phenomenon in the American Mineralogist in 1931 (Pabst 1931). He reports that pressure shadows are usually composed mostly or entirely of quartz but can be composed of carbonates, chlorite, or talc. The fibers of quartz are always elongated perpendicular to the adjacent pyrite crystal face; however, the fibers bear no particular orientation to the crystal axis directions of the quartz grains. The pressure shadows are thickest in the direction of stretching of the rock during regional metamorphosis.

In this model, the pressure shadows form long before the rocks are exposed and begin to weather, so alteration of the pyrite has nothing to do with their formation. Why, then, did John only find pressure shadows associated with altered pyrite? If my proposed explanation for their origin is correct, this is the new mystery! Perhaps the matrix with the unweathered crystals is subtly different from the matrix with the weathered crystals, and for that reason pressure shadows did not form. Perhaps such a difference may also help to explain why some of the rocks and their enclosed pyrite crystals have weathered and others have not. Perhaps further searching would turn up fresh pyrite crystals with quartz pressure shadows around them.

Caption: Pyrite crystal in phyllite matrix showing fibrous quartz pressure shadows along the lower right and upper left faces, which are still partially embedded in the matrix. The lower left face, still embedded, has no pressure shadow, nor does the face opposite it, which is not visible in the photograph. A pressure shadow from another crystal is visible in the background. The specimen is from Route 10 east of St.-François River, Sherbrooke, Quebec, Canada. Pyrite is 20 mm on edge. R. Peter Richards specimen no. 1280 (collected 1986) and photo.

REFERENCE

1. Pabst, A. (1931) “Pressure shadows” and the measurement of the orientation of minerals in rocks. American Mineralogist 16, pp. 55-70.

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