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A greenish yellow zoisite

Author: Gagan Choudhary

(This article was first appeared in The Australian Gemmologist, Vol. 24, No. 4, pp 90 - 91)

Zoisite is principally known for the magnificent purple-violet-blue coloured variety referred to as ‘tanzanite’. Due to the limited natural occurrence of this variety, much of the material available in the marketplace has been derived by heating common green-brown (khaki-coloured) zoisite. In addition to these, other varieties such as the chromium-coloured bright green to bluish green and yellowish green zoisite (Barot and Boehm, 1992), massive pink thulite, particoloured purple-pink and greenish yellow (Wentzell, 2000) and rare red zoisite (Koivula and Kammerling, 1991) have been documented in the literature.

The author encountered an interesting and unusual greenish yellow specimen (figure 1) that was subsequently identified as zoisite. A transparent pear-shaped specimen weighing 8.33 carats and measuring 14.87 x 11.89 x 7.80 mm was submitted for identification to the Gem Testing Laboratory, Jaipur, India.

Initial observations, particularly the colour, suggested that the stone was a tourmaline, but on rotation in all directions the stone displayed a strikingly strong pleochroism not often associated with tourmaline. The pleochroism was confirmed with a dichroscope and three colours were seen, namely yellow, blue and purplish pink. This pleochroism (trichroism) became even more interesting when viewed through a polarising filter. The colour of the specimen changed from yellow to blue and purplish pink on rotating the polarising filter to 90 degrees (figure 2). Both blue and purplish pink colours were visible at the same time; the blue was seen towards the tip of the culet and the purplish-pink colour towards the table of the stone. This was due to the difference in the line of transmission of the wavelengths in the two areas, governed by the inclination of the pavilion facets.

Figure 2: The zoisite specimen in Figure 1 displayed a strikingly strong trichroism. The specimen colour changed from yellow (left) to blue and purplish pink (right) on rotating the polarising filter to 90°. Both blue and purplish pink colours were visible at the same time; the blue was seen towards the tip of the culet and the purplish pink colour towards the table of the stone. This was due to the difference in the line of transmission of the wavelengths in the two areas, governed by the inclination of the pavilion facets.

Standard gemmological testing was performed to establish the identity of this specimen. Refractive indices (RI) were measured at n1.695 – n1.703, with a birefringence of 0.008. Hydrostatic specific gravity (SG) was calculated at 3.35. No absorption features were observed using a desk-model spectroscope and the specimen was inert under both SW and LW UV light. These properties (RI and SG) are consistent with those reported for zoisite. Under magnification, fine long tubes and cleavage planes were noted; these features were not considered to be of significance in this instance.

Although the specimen was identified by standard gemmological tests, this was confirmed by the FTIR spectrum. The spectrum (figure 3) displayed strong peaks at around 4337 and 4015 cm-1, a broad absorption band from 3800 to 2650 cm-1 and complete absorption of wavelengths from 2300 to 400 cm-1 with many small peaks at around 5423, 5176, 4822, 3876, 3850, 2480 and 2376 cm-1. This spectral pattern exactly matched to those of tanzanite and a few brown-coloured zoisites from our database. Qualitative EDXRF analysis revealed the presence of Al, Si, Ca and Sr as major elements with traces of V and Fe.

Figure 3: FTIR spectrum of greenish yellow zoisite displayed absorption features consistent with tanzanites

Therefore, on the basis of its RI, SG and FTIR spectrum, this greenish yellow specimen was conclusively identified as zoisite. This was the first time the author has encountered this very unusual colour shade of zoisite. However, yellow and yellow-green varieties have been mentioned previously (Barot and Boehm, 1992; Pearson, 2008; Webster, 1994; Wilson et al., 2009) but no images were found during the literature search such as those contained in this report. Colour variants such as this for zoisite provide additional options for use in jewellery.

References

Barot N.R. and Boehm E.W. (1992) Gem Quality Green Zoisite, Gems & Gemology, 28(1), pp. 4-15.

Koivula J.I. and Kammerling R.C. (1991) Unusual red zoisite, Gem News, Gems & Gemology, 27(3), pp. 185-186.

Pearson G. (2008) A spectrophotometric study of the thermal colour change of tanzanite, The Australian Gemmologist, 23(6), pp. 254-265.

Webster R. (1994) Gems, 5th ed., revised by P. G. Read, Butterworth-Heinemann, Oxford, UK, pp. 387-388.

Wentzell C.Y. (2000) Bicoloured zoisite, Lab Notes, Gems & Gemology, 36(2), pp. 159.

Wilson W.E., Saul J.M., Pardieu V., Hughes R.W. (2009) The Merelani Tanzanite mines, The Mineralogical Record, 40(5), pp. 347-408.

All photographs and photomicrographs by Gagan Choudhary