This 10.14 ct natrolite crystal supposedly originating from Mwanyanga, near Comolo, Arusha, Tanzania was donated by Kilimanjaro Gemstones.
Natrolite is a tectosilicate mineral species belonging to the zeolite subgroup. It is a hydrated sodium and aluminium silicate with the formula 'Na2Al2Si3O10 · 2H2O'.
The crystal has the form of a slender square prism terminated by a low pyramid, the prism angle being nearly a right angle. It is tetragonal in appearance, though it really is orthorhombic. It likely comes from a divergent tuft where the crystals are slender and are aggregated in.
Figure 1. 10.14 ct Tanzanian natrolite crystal| Shape | long thin (slender) square section prism crystal |
| Size | 5.3 x 5.3 x 37.8 mm |
| Color | colorless |
| Lustre | vitreous |
| Weight | 10.14 ct |
| SG | 2.25 |
| RI | ~ 1.495 (max) |
| DR | not measurable |
| Pleochroism | not applicable |
| Polariscope / Conoscope | light and dark four times through 360° observed from (xx0) facets (_|_ crystal C-axis) |
| SWUV | inert |
| LWUV | inert |
| Magnetic susceptibility | very slightly diamagnetic |
Table 1. Observational and measured properties
Infrared reflectance spectroscopy:
Figure 2. IR reflectance spectrum of the 110 crystal face.
The spectrum of the 110 crystal face, figure 2, is consistent with the spectra of other natrolite from different locations (see Zeolite group spectra). The spectrum of the 1-10 face does show any difference with the 110 spectrum.
Conclusion:
This 10.14 ct crystal supposedly coming from Tanzania has gemological and spectroscopic properties consistent with natrolite.