Black opal from Honduras

Opal-bearing volcanoclastic rocks from Gracias/Honduras were investigated u sing XRF, SEM,, electron microprobe, FTIR, UV-VIS and XRD techniques. The r esults demonstrate structural and chemical heterogeneities of the whole roc k on a small scale. The groundmass is mainly composed of glass shards, vesi cles and pumice lumps. The chemical composition of the shards and vesicle s hells indicates that the glass originated from a rhyolitic magma. Transpare nt yellowish opal with or without play of color occurs in interstices of th e glass shards whereas translucent precious opal with play of color fills f ormer steam vesicles. Both types of opal are similar to opal-A based on FTI R 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 acc umulated by non-precious opal of the groundmass and pumice lumps. They coul d not be detected in the precious opal substance within the former vesicles . According to the typical Soret band near 400 nm and the observed combinat ion of absorption bands in the W-VIS spectra, the presence of a porphyrin-l ike compound is suggested. The ignimbrite was probably infiltrated by silic a-rich and porphyrin-bearing fluids thought to be derived from adjacent car bonaceous sediments and diatomites. Porphyrins are suggested to render the black color of the opal-bearing whole rock.