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March-April 2013

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Fluorapatite From A Remarkable Occurrence of Graphite And Associated Minerals

Although small in number, apatite crystals found in the December 2007 pocket in the Karo pit, Block D, Merelani Hills, Tanzania (see accompanying article by Jaszczak and Trinchillo 2013) exhibit noteworthy paragenesis, inclusions, and extraordinary optical properties. These features along with recent analytical data are described herein.

The apatite from this find is fluorapatite, Ca5(PO4)3F. It is found in transparent (gemmy), elongate to equant crystals with pyramidal and pinacoid terminations (figs. 1 and 2). The color of each specimen ranges from yellow-gray to pale blue to green to dark blue, depending on the type of incident light, polarization of light, and orientation of the crystal.

Supplementary material for this article:

  1. Experimental details and apatite crystal structure refinement (download)
  2. A short movie showing the strong pleochroism in the fluorapatite from the Karo mine, Merelani Hills, Tanzania. The specimen is pictured in Figure 9 of Long et al. (2013). The movie was taken with a polarizing filter on the camera lens. It starts with the lens polarization direction parallel to the c-axis, [001] of the apatite. The polarizer is rotated 180o as the movie progresses. John Jaszczak, video; John Rakovan, audio. (download)
  3. This animated gif file shows the appearance of the multi-phase inclusions (fluids + graphite) within the fluorapatite crystal, pictured in Figure 2 of Long et al. (2013), when focusing a microscope through different levels of depth in the crystal.  The graphite-bearing inclusion at the center of the image is approximately 230 microns across. The image is comprised of eight different images taken at different focal planes. Movie created by John Jaszczak. (download)

Jena M. Long is a recent graduate from Miami University in Oxford, Ohio. This study was part of her undergraduate thesis research.

Dr. John Rakovan, an executive editor of Rocks & Minerals, is a professor of mineralogy and geochemistry at Miami University in Oxford, Ohio.

Dr. John A. Jaszczak, adjunct curator of the A. E. Seaman Mineral Museum, is a professor of physics at Michigan Technological Universiy in Houghton, Michigan.

Dr. Andre J. Sommer is a professor of chemistry at Miami University in Oxford, Ohio.

Dr. Robert Anczkiewicz is a research scientist at the Institute of Geological Sciences, Polish Academy of Sciences in Krakow, Poland.

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