Spec4Gem - Apatite sold as green Euclase from Brazil

Details: Created: 08 November 2015

The gemstone was sold as a green euclase from Brazil with a "authenticity certificate". Green euclase does exist and is rather rare compared to the colorless or the rare blue euclase.

A quick observation of the gemstone rose some doubts about the exactness of the material that were confirmed by the classical gemological tests. The results are sumarized in table 1. The RI—1.641-1.650 is rather low compared to the known data of euclase given for 1.649-1.676. The birefringence wich is only 0.009 is also too low against the 0.017-0.025 of euclase. The uniaxial optical character is not compatible with the biaxial optical character of euclase. Measured hydrostatically, SG—3.27 is higher than that of euclase, usually between 2.99 and 3.15. The desk-model spectroscope did not reveal any absorption.

The most common gemstone matching the observed properties is apatite.

Figure 1. 1.47ct Apatite reportedly from Brazil sold as green euclase

Shape	emerald / step cut
Size	8,7 x 5,8 x 3,6 mm
Color	greenish-yellow
Lustre	just vitreous
Weight	1.47 ct
SG	3.27
RI	1.641 - 1.650, Uniaxial negative
DR	0.009
Pleochroism	strong greenish-yellow / gray
Polariscope / Conoscope	anisotropic / interference figures partially observed - optical character couldn't be determined
SWUV	inert
LWUV	inert
Observational features	facet edges are not very sharp but slightly rounded with few chips, hardness is likely below 7...

Table 1. Observational and measured properties

Infrared reflectance spectroscopy:

Infrared reflectance spectrum was acquired from the table of the sample without any particular orientation.

Figure 2. IR reflectance spectra of the studied sample (red), an colorless euclase from Brazil (blue) and yellow apatite crystal from Brazil (cyan).

The comparison of the studied sample spectrum to a spectrum of an apatite crystal from Brazil and to an colorless euclase from Brazil (figure 2), yield the conclusion that the material of the studied sample is apatite, a phosphate mineral with its very distinct spectrum.

Photoluminescence spectroscopy:

The gemstone is inert to the exposure to the short- and long-wave UV lamp, but a 405 nm 1 mW laser pointer produces a weak orangy luminescence. The photoluminescence spectrum was acquired with a 405 nm laser with a power of 20 mW. The laser ray was oriented approximately parallel and perpendicular to optical axis with no significant difference in spectra.

pl405 apatite147 ex euclase Figure 3. 405 nm excitation photoluminescence spectrum of the 1.47ct gemstone showing Mn²⁺ and Sm³⁺ replacing Ca²⁺.

The spectrum (figure 3) in the 450-1000 nm range shows a main wide peak around 588 nm to which are added three narrow peaks at 567, 600, 646 nm. Such a spectrum is typical of the REE spectra in fluoraptite ^[1], especially with Sm³⁺ ions at 567, 600, 646 and 708 nm. The 588 nm peak is also well-known in apatite ^[1] and it is attributed to Mn²⁺ ions replacing Ca²⁺. Such kind of luminescence spectra are unknown for euclase that usually shows a quite strong and broad emission peak around 750 nm as a result of the Fe³⁺ luminescence where Fe³⁺ replaces Al³⁺.

pl254 apatite147 ex euclase Figure 4. 254 nm excitation photoluminescence spectrum of the 1.47ct gemstone possibly showing Eu²⁺.

The spectrum obtained with a 254 nm excitation source (figure 4) has a weak signal due to the low power of the source. Despite that, a continuous emission starting from the red around 650 nm to the violet around 410 nm with a maximum at 446 and 468 nm. The result of all these emissions should produce a bluish white photoluminescence but in this particular case it is too weak to be detected with the eyes. After Gorobets and Alexandre A. Rogojine ^[2], such kind of emission could possibly be attributed to Eu²⁺ in fluorapatite. Euclase can produce photoluminescence with a very broad emission peak centered around 370 nm attributed to O* ^[2] which is not comparable to the emission observed in this spectrum.

Discussion:

After the classical gemological tests and observational features, IR reflectance spectroscopy and photoluminescence spectroscopy, the 1.47 ct yellowish-green gemstone is an apatite but it is not an euclase as reported.

^[1] Luminescence Spectroscopy of Minerals and Materials, M. Gaft, R. Reisfeld, G. Panczer, Springer Editor, ISBN: 103540219188

^[2] Luminescent Spectra of Minerals, Boris S. Gorobets and Alexandre A. Rogojine, Moscow, 2002, ISBN: 5901837053