Gem News International Gems & Gemology, Spring 2013, Vol. 49, No. 1

Zoned Scapolite from India

The Spring 2011 GNI section (p. 59) reported gem-quality yellow scapolite from Karur in the southern India state of Tamil Nadu. While visiting a local dealer, this contributor encountered parcels of rough and faceted scapolite that appeared strongly zoned to the unaided eye. They were reportedly mined from the same region as above. From the parcels, one rough and one faceted specimen were obtained. The faceted stone was a 9.10 ct oval mixed cut, while the 4.18 g rough specimen was a tetragonal prism with partially broken bipyramidal terminations (figure 1). Both had a pale yellow color with strong brown zones.

Figure 1. These strongly zoned scapolites, a 9.10 ct faceted stone and a 4.18 g rough specimen, were reportedly mined in the Karur region of Tamil Nadu in India. Note the patchy appearance of brown zones. Photos by G. Choudhary.
Standard gemological testing on the faceted sample revealed an RI of 1.548–1.570, with a uniaxial negative optic sign and a birefringence of 0.022; its hydrostatic SG was measured at 2.66. These values are consistent with those reported for scapolite. Both samples fluoresced strong orange-pink (almost red) under short-wave UV radiation and were inert to long-wave UV. No absorptions were seen with a desk-model spectroscope. The color zones appeared patchy and showed some flaky inclusions along certain planes. This was confirmed with higher magnification (figure 2, left). Under oblique illumination these planes appeared highly iridescent, displaying bright spectral colors.

SP13-GNI-17-AB 147428
Figure 2. The color zones were composed of fine platelets oriented in planes along the length of crystal (left). Under oblique illumination, these planes displayed bright interference colors and angular features (right), suggesting the exsolution of some mineral. Photomicrographs by G. Choudhary; magnified 64×.
Further observation at high magnification revealed some angular features. Although the planes intersect with each other at 90º, the features within them appeared to be following the bipyramidal faces, as indicated by their angle and orientation (figure 2, right). This brown mineral was found along planes in two directions intersecting at almost 90º (figure 3), which were oriented along the length of the crystal (its c-axis). Examination of the rough specimen revealed that these brownish planes were oriented along the prism faces, following the cleavage planes. Some of the brownish inclusions were also aligned along the basal pinacoid planes. The overall pattern and orientation of the zones indicated that some mineral has been exsolved along the cleavage planes, following the bipyramidal faces, as suggested by the angular features described above.

GNI 147566
Figure 3. The planes containing a brown exsolved mineral were present in two directions and intersected at almost 90º along the prism faces, which is also the direction of cleavage in scapolite. Photomicrograph by G. Choudhary; magnified 64×.
These brown platelets could not be conclusively identified, but Raman spectra indicated the presence of lepidocrocite, which has previously been reported as fillings in lath-shaped cracks of scapolite (E.J. Gübelin and J.I. Koivula, Photoatlas of Inclusions in Gemstones, Vol.1, ABC Edition, Zurich, 1997, pp. 368–369). While the Spring 2011 GNI entry noted the occurrence of gem-quality yellow scapolite from the same location, this production is probably more suitable as a collector’s stone.

Gagan Choudhary
Gem Testing Laboratory
Jaipur, India