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A historically important deposit of chrysoprase and prase opal in Tanzania is located at Haneti, situated north of Dodoma in central Tanzania (Shigley et al., 2009).
The Raman spectra shown in Figure 10 are very similar to the spectra of prase opal presented by Shigley et al.
EDXRF chemical analyses (with an EDAX Orbis Micro-XRF Analyzer) showed large variations in the Ni content of this prase opal. Analyses of three random spots (300 pm beam diameter) on both samples gave NiO contents of 3.64, 7.36 and 9.72 wt.% for the smaller opal, and 0.73, 3.08 and 5.09 wt.% for the larger piece.
The high Ni content of this strongly coloured prase opal appears to be consistent with the notion that chrysoprase with a lower degree of crystallinity tends to have greater amounts of Ni and a more intense colour (cf.
It is highly likely that this prase opal was found in the same belt of metamorphosed ultramafic rocks that, from Haneti, are aligned in a north-westerly direction along regional shear zones.