Fig. 4

Microscopic characterization of the carbonate scale-fragment fabric. a Reflected light (RL) cross-section showing a basal sulfide corrosion layer (red arrows), columnar Mg-calcite crystals, and rhombohedral crystal tips forming a rough scale growth surface (blue arrows). b RL image showing a layer of detrital particles (variable thickness, yellow arrows) on the former calcite growth surface (blue arrow) and subsequent re-nucleation of numerous calcite crystallites (white arrows). c Typical metal sulfide layer of variable thickness at the scale-fragment base (red arrow). d Transmitted light (TL) image of basal corrosion layer being an attractive substrate for abundant calcite nucleation and ongoing competitive crystal growth. e Scanning electron microscope (SEM) image of rough scale growth surface showing a dense fabric of rhombohedral calcite crystals. f Back-scattered electron (BSE) image illustrating typical metal sulfide layers at the scale-fragment base (partially fractured, red arrow) and as thin layers within the compact carbonate scales (yellow arrows)