Revisiting Natural “Opals” from Greece: Insights from Northern Evia Island and the Occurrence of Moganite

Natural silica materials traditionally described as “opal” encompass different structural states, and their macroscopic appearance does not necessarily reflect their crystallographic identity. This study re-examines “opal”-like materials associated with ultramafic-hosted magnesite deposits of northern Evia Island, Greece. PXRD, ICP-MS, μ-XRF, micro-Raman spectroscopy, solid-state 29Si MAS NMR, and TEM were combined to determine their mineralogical identity, structural state, and colour-related mineralogical characteristics. The investigated materials are predominantly poorly ordered to crystalline rather than truly amorphous, with α-quartz as the principal SiO2 phase. The faintly green EVIA-4 material is predominantly quartz and contains 661 ppm Ni, preferentially associated with sepiolite-rich domains, suggesting that finely dispersed Ni-bearing sepiolite is the most likely mineralogical contributor to its colour. Moganite was not detected, and EVIA-4 is more appropriately regarded as quartzite rather than opal. In contrast, the reddish to pale-pink EVIA-5 material consists predominantly of α-quartz with moganite identified by PXRD and micro-Raman spectroscopy. 29Si MAS NMR and HRTEM/SAED further confirm its crystalline and nanocrystalline character. EVIA-5 is therefore more appropriately classified as chalcedony, likely carnelian, and represents, to our knowledge, the first occurrence of moganite associated with an ultramafic-hosted magnesite deposit in Greece.