4.3. Morphology and distribution of ferruginous minerals
Ferruginous mineral with diameter ranging from hundreds of nanometers to tens of microns are randomly scattered or gathered in ooids (Figs. 6, 7). Two distinguishing morphologies, euhedral crystals and aggregated spheroids, of these minerals are identified through SEM-EDS examination. Euhedral crystals are generally found in the calcium nuclei of ooids, and usually range from several to tens of microns (Fig. 6a-c). They display regular five to six-sided edges in polished thin sections (Fig. 6d). In several cases, this type of iron oxide depicts concentric zones that differ in thickness, size and possibly in chemical composition (Fig. 6e, 6f). The inner zones have irregular edges in contrast with regular six-sided margin of the outer zones (Fig. 6f). Spheroid aggregates randomly occur in the whole ooids (Fig. 7), and can reach to a high proportion. Every single aggregate consists of fine amorphous ferruginous spheroids with diameters around or lesser than 1 μm (Fig. 7c). The arrangements of the individual ferruginous spheroids are mostly disordered. What is more, at or near spheroid-to-spheroid contacts, there are bridge-like iron oxides exhibiting different brightness with spheroids (Fig. 7c).
The ferruginous minerals occur mostly in ooids and rarely in cements (Figs. 6, 7). Both euhedral crystals and aggregated spheroids distribute randomly in ooids, they can form a whole concentric layer or not (Fig. 7a). The amount and occurrence of them in different ooids are also random. In ooids with quartz nuclei, ferruginous mineral is absent in the nuclei but scatters among the cortex (Fig. 7b, 7d). However, in ooids with calcium nuclei, ferruginous mineral occurs in whole ooids (Fig. 6a, 6b).
Interestingly, in SEM-EDS analysis, ferruginous mineral shows enrichment in S (over 30 cps/0.10 wt%) content relative to surrounding matrix, which contains no more than 30cps/0.05 wt% S (Fig. 8).