Black opal from Honduras
Banerjee, Arun; Wenzel, Thomas
European Journal of Mineralogy Volume 11 Number 2 (1999), p. 401 - 408
published: Apr 19, 1999
manuscript accepted: Nov 26, 1998
manuscript received: Sep 26, 1997
ArtNo. ESP147051102019, Price: 29.00 €
Abstract Opal-bearing volcanoclastic rocks from Gracias/Honduras were investigated using XRF, SEM, electron microprobe, FTIR, UV-VIS and XRD techniques. The results demonstrate structural and chemical heterogeneities of the whole rock on a small scale. The groundmass is mainly composed of glass shards, vesicles and pumice lumps. The chemical composition of the shards and vesicle shells indicates that the glass originated from a rhyolitic magma. Transparent yellowish opal with or without play of color occurs in interstices of the glass shards whereas translucent precious opal with play of color fills former steam vesicles. Both types of opal are similar to opal-A based on FTIR transmission spectra and XRD results. The high total carbon content of 1.6 wt.% of the whole rock is caused by organic carbon compounds as indicated by C=O and C-H groups of FTIR spectra. The carbon compounds are mainly accumulated by non-precious opal of the groundmass and pumice lumps. They could not be detected in the precious opal substance within the former vesicles. According to the typical Soret band near 400 nm and the observed combination of absorption bands in the UV-VIS spectra, the presence of a porphyrin-like compound is suggested. The ignimbrite was probably infiltrated by silica-rich and porphyrin-bearing fluids thought to be derived from adjacent carbonaceous sediments and diatomites. Porphyrins are suggested to render the black color of the opal-bearing whole rock.