Spec4Gem - Ophite from Urals mountains, Russia

Ophite from Urals mountains, Russia

Details: Created: Monday, 31 December 2018 10:58

The dark-grayish-green ophite from Urals mountains in Russia is a rock composed of three mineral species which belong to the serpentine group: antigorite, chrysotile and lizardite. This material is sometime named 'ophite jade' or 'serpentine jade' because of its color. The dark-grayish-green cabochon (figure 1) shows a homogeneous color with some white veins, the material is semi-transparent while the tickness is not too tick.

With a quite strong light it is possible to observe by transparency some darker green veins with surrounding reddish areas. This phenomenom has already been observed in Cr- antigorite and bowenite showing the usambara effect, here the phenomenom is rather weak. The gemstone is viewed red through the chelsea filter if the stone is backlighted with an incandescent light.

serpentine ophite 7000 dark grayish green UralsMountains Russia

Figure 1. The 70 ct cabochon of dark-grayish-green ophite from
Urals mountains, Russia, shows some whitish areas and few white
thin veins.

Shape	flat rounded corners rectangular cabochon
Size	45.6 x 32.1 x 4.05 mm
Color	dark-grayish-green shades with some white veins
Lustre	greasy
Weight	~ 70 ct
SG
RI	-
DR	-
Pleochroism	-
Polariscope / Conoscope	stays light
SWUV	the white veins are white and the green material is inert
LWUV	the white veins are strong white and the green material is inert
Magnetic susceptibility N52	quite strong, the N52 magnet drags the stone
Chelsea filter	inert in reflected light while it is red in transmitted incandescent light from the back

Table 1. Observational and measured properties

Infrared reflectance spectroscopy:

The IR reflectance spectrum (figure 2) was collected from the cab's top in a green area. The reflectance equipment used to collect does not allow to get the reflected IR from the white veins since they are embbeded in the green material and they cannot be isolated. The spectrum's pattern is characteric of serpentines. It is not worth to speculate in identifying the three serpentines species (antigorite, chrysotile and lizardite) within the spectrum. The 568 cm^-1 band generally attributed to Mg-O is characteristic of antigorite.

irs serpentine ophite 7000 dark grayish green UralsMountains Russia

Figure 2. IR reflectance spectrum of a green area of the ophite cab showing a characteristic serpentine pattern.

UV-VIS-NIR spectroscopy:

The UV-Vis-NIR spectrum (figure 3) was collected from a green area of the cab with the light path travelling from the top to the back of the cab. It shows a hypothetical strong absorption around 400 nm followed by another quite stong band at 572 nm. The asymetry of the 572 nm band suggest another band around 630 nm. A small feature at 685 nm is possibly connected to Cr³⁺, but it is not clear here if it is an absorption band or a luminescence artefact. There is a weak and large band centered at 730 nm followed by another large band peaking around 905 nm. As in most of the serpentines, the triplet at 946, 954 and 961 nm is well visible in the spectrum.

Ni²⁺, Fe²⁺ serpentines show large absorptions in the NIR range, except the 730 nm band, the almost unexistant 905 nm band, the gemstone is likely not colored by iron and nor nickel even if they can be present. The 572 nm and the secondary 685 nm small feature likely indicates a possible coloration by Cr³⁺. The too strong absorption in the violet cannot be collected because of spectrophotometer's limits, but as written earlier, the presence of the hypothetical strong absorption around 400 nm could confirm the Cr³⁺ as the color cause.

uvvis serpentine ophite 7000 dark grayish green UralsMountains Russia

Figure 3. The UV-Vis-NIR spectrum collected from a dark-green area shows two humps, one hypothetical around 400 nm and the second one at 572 nm likely attributed to Cr³⁺ as well as the 685 nm one. The weak features at 630, 730 and 905 nm are possibly related to Fe²⁺ and the triplet at 946, 954 and 961 is still to be discovered.

The transmission window in the green and the quite high transmission in the red and deep-red can explain the weak observed usambara effect and the red color of the stone if backlighted with an incandescent light.

Photoluminescence spectroscopy:

The luminescence spectroscopy was performed with the 405 nm laser, two areas were investigated, a green area and a white vein, the spectra are shown in figure 4. The green area shows a red luminescence with a large emission centered around 730 nm which is decorated with three small peaks at 692, 708 and 727 nm. Such red emission is quite characteristic of Cr³⁺. The white vein shows a white luminescence as observed with SWUV and LWUV lamps, which is characterized by a large emission between 400 and 650 nm with a maximum around 510 nm. Could it be organic molecules luminescence, further studies have to be conducted to understand it, knowing the nature of the white material might easily explained it.

pl405 serpentine ophite 7000 dark grayish green UralsMountains Russia

Figure 4. The photoluminescence spectrum of the green area shows Cr³⁺ red emission (692, 708, 727 nm). The white veins show a large white emission between 400 and 650 nm peaking at 510 nm, this emission is unexplained at the moment.

Conclusion:

The green material is serpentine as confirmed by IR spectroscopy but the mixture of the three serpentine varieties entering in the composition (antigorite, chrysotile and lizardite) has not been investigated. The white veins material was not investigated as well and it is still unknown. The color is likely caused by Cr³⁺ as shown by UV-Vis spectroscopy and those presense is confirmed by the red photoluminescence. Fe²⁺/Ni²⁺ are possibly present but likely in weak concentration and do not affect the NIR spectrum. The Cr³⁺ and Fe²⁺/Ni²⁺ can also explain the gemstone strong reaction to the N52 magnet.

This ophite material from Russia is visually interesting although is not often seen in the market.