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The origin of opal-bearing listvenite from the Tokat region (northern Turkey)
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DOI：10.1180/mgm.2025.10174.png)
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Abstract
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This study reveals the mineralogy and origin of unusual silica-carbonate bodies (listvenite) associated with tectonized serpentinites and Permian–Triassic phyllites from the Tokat region in northern Turkey. The listvenite includes green-coloured opaline silica intersected by saddle dolomite and minor sulfide mineralization (pyrite; cobaltite and enargite). It also hosts abundant chalcedonic bodies; i.e. agate of bluish to bluish-white colour. Listvenite-hosted opaline silica has been recognized as lussatite; a fibrous birefringent opal-CT variety; with a dominantly ‘tridymitic’ structure. The green colouration results from discrete; but widely disseminated; Cr-bearing smectite-group minerals; such as volkonskoite and/or Cr-montmorillonite. Chromium; the main colouring agent as determined by element-distribution mapping and visible-light spectroscopy; was probably derived from spinel-group minerals (e.g. chromite); the relics of which mark the presence of an ultramafic protolith. In contrast; the vein agate is composed of length-fast chalcedony; with subordinate quartzine and minor opaline silica; followed by drusy quartz. Its bluish colour appears to be related to scattering effects enhanced by elevated moganite contents (up to 46 wt.%). The observed mineral paragenesis suggests at least a two-stage evolution: (1) early listvenitization marked by carbonate-silica alteration of the host serpentinite together with concomitant sulfide precipitation; and (2) subsequent silicification producing chalcedony (agate) veins under low-temperature conditions; followed by possible dissolution of the earlier saddle dolomite. This evolution reflects complex fluid–rock interactions involving CO2-rich; Ca-bearing and later Si-rich fluids under fluctuating redox and pH conditions. Hence; the proposed crystallization sequence of the associated mineral assemblage is: sulfides (cobaltite+pyrite+enargite) → saddle dolomite → opal (lussatite) + Cr-smectite → chalcedony (agate). These data provide new insights into the genesis; colouration mechanisms and structural state of Cr-bearing opaline and chalcedonic silica in altered ultramafic terrains.
Keywords:
chrome opal
lussatite
agate
chalcedony
saddle dolomite
listvenite
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