Resolving colouration mechanisms in gem-quality chalcedony: mineralogical and spectroscopic constraints

Abstract

The mineralogical and spectroscopic constraints on gem-quality chalcedony-rich bodies from South Khorasan Province (Eastern Iran) were provided to determine their origin and colouration mechanisms. Four distinctive colour varieties, i.e. (I) green, (II) bluish, (III) composite brownish-violet and white, and (IV) nearly colourless with discrete agate banding, were analysed using Raman (RS), Fourier-transform infrared (FTIR), and optical absorption spectroscopy, supported by cathodoluminescence (CL) microscopy and X-ray powder diffraction (XRPD). Type-I was formed via in-situ silicification (birbiritization) of a serpentinized ultramafic protolith (Cretaceous Birjand ophiolite), while other species (type II-IV), hosted by Cenozoic andesitic rocks, originated from post-volcanic hydrothermal activity. The green colour of type-I chalcedony was facilitated by minute Cr-bearing phyllosilicates (possibly smectite-group) that give a specific absorption at 683 nm in the optical absorption spectrum. The bluish hue of chalcedony (type-II) is attributed to light scattering effects enhanced by moganite enrichment, whilst brownish-violet colour (type-III) results from the presence of inclusions (e.g. hematite, carbonaceous material, and dolomite), as well as the possible presence of Fe-related colour centres typical of amethyst. Meanwhile, white regions recognized in both type-II and type-III chalcedony are rich in discrete sepiolite-palygorskite inclusions and/or comprise a peculiar “transitional” phase with a mixed spectroscopic signature of opal, α-quartz, and moganite, as well as a low crystallinity index (CI) of quartz of 1.30. This phase highlights an ongoing textural maturation of amorphous silica. Colourless chalcedony (type-IV) features the peculiar opal-CT-rich layer at the boundary with host andesite. Furthermore, the opal encloses abundant zeolite-group species (Na-heulandite/Na-clinoptilolite) that not only exhibits an unusual bluish-green CL emission, but also formed due to the interactions of agate-forming fluid with the groundmass of volcanic rock.