Green petrified wood from Gokwe, Zimbabwe
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- Created: Tuesday, 24 December 2024 10:58
The Gokwe South District in the Midlands, Zimbabwe, hosts a petrified forest, where nice logs of worldwide rare Woodworthia and extremely rare Rhexoxylon were found. The Woodworthia, a primitive conifer, is a lovely green colour with typical bark as in the slice cut from a trunk section shown in Figure 1. The second specie found there is the Rhexoxylon, a tree fern, a fantastic with grey, orange and pink shades with the typical large canals. These woods are believed to be from the Triassic epoch, around 200 million years ago.
Green petrified woods are relatively rare amongst all available ones, some are also found in the USA, mainly in Arizona.
Petrified woods are also described as fossil wood.
Photographies from the Figure 2, 3, 4 and 5 give some details of the slice (wood anatomy, luminescence and resin filling).
Figure 1. The slice of green petrified wood measures 254 x 197 mm,weights 1,750 g and shows different hue of green, some kakis,
browns and some grays. Bark parts are visible on the top-right
'corner'.
| Shape | slice of a trunk section |
| Size | 254 x 197 x 18 mm |
| Color | greens, browns, grays |
| Diaphaneity | opaque |
| Lustre | vitreous |
| Weight | 1,750 g |
| SG | - |
| RI | - |
| DR | - |
| Pleochroism | - |
| Polariscope / Conoscope | - |
| SWUV | yellow in the brown areas but weaker than the LWUV, inert in the greens |
| LWUV | yellow in the brown areas, inert in the greens |
| Magnetic susceptibility N52 | - |
| Chelsea filter | slightly pinkish |
Table 1. Observational and measured properties
Figure 2. Xylem anatomy with almost rectangular tracheids' cross sections. Smartphone photography through the occular of the binoccular
at 20x.
Figure 3. The curved lines seem to be intergrowth rings between early and late wood. Smartphone photography through the occular of the
binoccular at 20x.
Figure 4. Long wave UV (365 nm) yellow luminescence of the brown area. Smartphone photography.
Figure 5. Left, open crack filled with a material containing bubbles, halogen light. Right, same filled open crack lighten by short wave UV
(365 nm) light and glowing bright blue. Such a blue luminescence is indicative of a resin filling Smartphone photography through the occular
of the binoccular at 20x.
Infrared reflectance spectroscopy:
The slice is too big to be set in the FTIR spectrometer!
Raman spectroscopy:
As for the FTIR, the slice is again too big to be set in the Raman setup, but we might expect a Raman spectra within a month!
UV-VIS-NIR spectroscopy:
The slice being opaque, the Vis-NIR spectrum in Figure 6, was collected by reflectance. It is the result of two broad and strong bands at 420 and 604 nm, the 420 nm band is along with a hump at 455 nm. Some additionnal and weaker features are present at 681, 686 and 945 nm. The 420 and 604 broad bands causes a transmission window around 500 nm responsible for the green color. These bands, as well as the 681/686 are associated to Cr3+ ions and their poistion can vary depending of the hosting mineral. The 945 nm band could be associated to an overtone of the OH/H2O bands in the IR range.
Figure 6. The spectrum collected from a green area shows a characteristic Cr3+ spectrum within minerals such as chalcedonies (quartz), beryls, corundums etc.Note that Vanadium shows similar spectra to Chromium ones and it is often difficult to distinguish them, but the 681 and 686 bands are mostly related to the Cr3+ spin-forbidden transitions, so even if some V3+ were present, the main contribution to the color is that of Cr3+. This is consistent with the chemistry analysis by LA-ICP-MS published by G. Mustoe and M. Acosta, 2016, where V is two hundreds times rarer than Cr. According to the same analysis results, iron (Fe) is present at a concentration about four times lower than Cr, could it contribute to the color? There is no strong evidence of Fe in this Vis-NIR spectrum, although the 455 nm band (here as a hump) can be related to Fe as in corundum i.e. but the Fe large and strong bands above 700 nm are missing. Lacking samples of other green petrified woods, it is difficult to compare with iron spectra in similar context.
Photoluminescence spectroscopy:
The photoluminescence spectrum was collected from the brown area with three distinct excitations: 365, 405 and 447 nm, all gave the same spectrum pattern as shown in Figure 7. The spectrum seems to be a composite spectrum with distinct emissions at 490, 565, 585 and 625 nm yielding a green-yellow to yellow emission. Such yellow emission could be ascribed to zircon, this is just a working hypothesis that needs to be deeply investigated.
Figure 7. The photoluminescence spectrum collected from the brown area with three distinct excitations (365, 405 and 447 nm) shows a composite large emission in the green-yellow to yellow.Conclusion:
Although this unusual green petrified wood slice does not obviously show wood structure it shows bright green colors. These greens are caused by the Cr3+ ions impurities in the host material likely chalcedony quartz (not confirmed by FTIR, Raman or Refractive Index).
[1] Origin of Petrified Wood Color, G. Mustoe and M. Acosta, Moscow, 2016, Geosciences, MDPI,
https://doi.org/10.3390/geosciences6020025.