Gems & Gemology, Winter 2016, Vol. 52, No. 4

Apatite Cluster in Orthoclase Feldspar

Jonathan Muyal and John I. Koivula
Apatite crystal cluster in feldspar.
Figure 1. Located just left of center, a large apatite crystal cluster is visible directly below the table facet of this 7.20 ct orthoclase feldspar (the inclusion is also reflected numerous times by the pavilion facets). Photo by Kevin Schumacher.

A yellow orthoclase feldspar (figure 1) recently examined by these authors was of particular interest, not only for its size and clarity but also for the unusual cut. The 7.20 ct oval had a wide table facet that dramatically framed a large eye-visible crystal cluster just below its surface. The potassium-rich orthoclase host was identified using traditional gemological testing and confirmed by Raman microspectrometry, which also identified the inclusion as apatite (figure 2).

Small crystal next to cluster.
Figure 2. A tiny crystal next to this large cluster of prismatic hexagonal apatite crystals resembles a space shuttle approaching a space station. The image was taken using darkfield illumination. Photomicrograph by Jonathan Muyal; field of view 4.79 mm.

This relatively large apatite cluster was composed of hexagonal elongated prismatic crystals, a morphology typical of the mineral. They also proved to be opaque and exhibited evidence of a certain degree of softness by their slightly corroded crystal faces (again, see figure 2). Several small tension stress cracks were observed surrounding the inclusion. The altered appearance suggests that these apatite crystals are protogenetic inclusions that were present in the growth environment before the orthoclase began to form.

Apatite, a common phosphate mineral, has been described in the literature as a crystal inclusion in various gem materials and as randomly scattered “jackstraw” needles in yellow orthoclase from Madagascar (E.J. Gübelin and J.I. Koivula, Photoatlas of Inclusions in Gemstones, Volume 2, Opinio Verlag, Basel, Switzerland, 2005). This specimen’s morphology and size make it an aesthetically pleasing example of apatite as a crystal inclusion in orthoclase, and therefore an interesting collector’s gemstone.

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