Recent results from Root at al. [42] on arsenic sequestration in high-iron sediments show from X-ray Absorption Spectroscopy (XAS) that "the HFO floc had been reduced to a mixed Fe(II,III) solid with a local structure similar to that of synthetic green rust (GR)", but also show that there is no evidence from XRD for the formation of a crystalline GR phase.
The magnesium-rich varieties commonly occur in ultramafic igneous rocks, the iron-rich varieties in metamorphosed iron-rich sediments.
Fayalitic olivines develop within metamorphosed iron-rich sediments.
The forsterite-fayalite minerals also occur in dolomitic limestones, marbles, and metamorphosed iron-rich sediments.
Hedenbergite is a product of thermal metamorphism of iron-rich sediments where its formation is probably due to the breakdown of actinolite with increasing temperature.
Hedenbergite is common in limestone skarns, in thermally metamorphosed iron-rich sediments, and in some igneous rocks; sodium-rich varieties are found in many alkaline rocks.
Hematite and magnetite, for example, are often found together in iron-bearing sediments; line ab of Figure 17 indicates the Eh and pH values at which their coexistence can take place.
These magnetite-apatite bodies encased in volcanic rocks have been variously interpreted as having formed as immiscible oxide magmas, as iron-rich sediments that were subsequently metamorphosed, and as deposits arising from volcanic exhalations.
The correlation between As and iron in sediment was found to be very poor.
The abundance of different chemical compounds present in the lake led the researchers to conclude that chemical reactions were taking place between the brine and the underlying iron-rich sediments, producing the nitrous oxide and molecular hydrogen.
(b) Normalized EXAFS and corresponding radial structure functions (uncorrected for phase shift of backscattering atoms) for iron in sediment samples from core SC4 (at 1.4 and 34.5 cm) compared to reference spectra of crystalline pyrite (FeS2 s)) and of iron substituted in reference illite IMt-1 (Clay Minerals Source Repository).
