Raúl Merinero

Nucleation, growth and oxidation of framboidal pyrite associated with hydrocarbon-derived submarine chimneys: lessons learned from the Gulf of Cadiz

In order to establish the history and conditions of nucleation and growth of pyrite formed inside methane-derived carbonate chimneys collected in the seafloor of the Gulf of Cadiz, we compared the size distributions, morphologies, textures and geochemical compositions of framboidal and euhedral pyrite as well as iron oxyhydroxides pseudomorphs. More than 2400 framboids and crystals were measured and four types of statistical populations were identified. Framboid size (diameter) ranges from 6 to 12.5 μm with a mean size of 10.4 μm and a maximum value of 27.9 μm, whereas euhedral crystal size (maximum dimension) ranges from 6.3 to 9 μm with a mean size of 7.2 μm and a maximum value of 21.6 μm. Crystal size distributions (CSD) of different populations of framboidal iron oxyhydroxides show strongly negatively skewed shapes, close to the universal steady-state CSD curve for Ostwald ripening, an unequivocal signal of closed-system growth. Euhedral crystal populations of pyrite and iron oxyhydroxides present CSDs with lognormal or slightly negatively skewed lognormal shapes and low size variances, indications of open-system growth. This was confirmed by simulating framboidal and euhedral crystal populations with different growth mechanisms using the GALOPER (Growth According to the Law of Proportionate Effect) program. From CSDs shapes and the values of α and β 2 , the populations of framboids are inferred to have grown initially in an open system (surface and then transport-controlled) and then in a closed system (Ostwald ripening) and the populations of euhedral crystals only in an open system. In this case, the initial stage of surface-controlled growth is shorter than that deduced for the framboidal populations. We suggest that: (1) the concentration and availability of reactive iron and hydrogen sulphide are the main factors controlling the morphology and growth of pyrite, and (2) the higher reactive surface of framboidal textures would govern their preferential oxidation, dissolution and recrystallization during the closed-system growth and consequently the development of higher sizes in comparison with euhedral forms.

Representative size distributions of framboidal, euhedral, and sunflower pyrite from high-resolution X-ray tomography and scanning electron microscopy analyses

Abstract A statistical procedure designed to obtain representative size distributions for different morphologies and arrangements of pyrite is described here. This statistical procedure is applied to data acquired during scanning electron microscopy (SEM) and high-resolution X-ray tomography (micro-CT) analyses. The statistical procedure was tested in methane-derived carbonate pipes recovered in the Gulf of Cadiz. These samples contain abundant pyrite together with pseudomorph iron oxyhydroxides showing multiple morphologies including euhedral crystals, framboids, and sunflowers (framboidal core with outer crystals). The SEM analysis consisted in the establishment of independent populations of pyrite and iron oxyhydroxides grouped by morphology and arrangement and the determination of its size distributions. Micro-CT analysis included a determination of the 3D volume of pyrite from the density difference between pyrite and the rest of mineral forming the samples. The use of the micro-CT technique implies that minerals with similar attenuation coefficients than those of pyrite are scarce or not present in the studied samples. A filtering process was applied to the 3D volume. This filtering process consisted of the selection of objects with corrected sphericity greater than 0.80, discrete compactness greater than 0.60, elongation and flatness of the circumscribed 3D ellipsoid less than 1.80 and the sum of the elongation and flatness less than 3. Objects with shapes similar to those expected in pyrite (spheroidal and regular shapes) were selected with this filtering process. The optimal mixture of lognormal size distributions was obtained applying statistical techniques to the entire size distribution represented by the filtered objects. The correspondence between size distributions obtained during the SEM and the micro-CT analyses was done by matching statistical parameters and using 3D renderings. The representative size distributions of pyrite as determined by the proposed 3D processing methodology can be used to accurately quantify the paleo-environmental conditions of pyrite formation, which would solve some of the limitations resulting from analyses based on 2D images.

Theoretical growth of framboidal and sunflower pyrite using the R-package frambgrowth

Framboids and sunflowers are the most ubiquitous shapes of sedimentary pyrite. Framboids are spherical aggregates of nanocrystals, while sunflowers are formed by overgrowth of framboids and represent intermediate stages in the transformation of framboids into euhedrae. The characterization of the size populations of these shapes provides critical information about the paleoredox conditions at time of formation and the subsequent changes in these conditions. This paper describes in detail an algorithm designed to model the growth and generate significant populations of both framboids and sunflowers, using functions of the statistical software R. The source code is provided as supplementary material to this paper. The algorithm uses several growth mechanisms based on dependence on or independence of the number of nanocrystals for framboids and the external diameter for sunflowers. Variability in the generated size populations depends on several parameters of the algorithm, such as the diameter of the nanocrystals, the initial diameter of the framboids and the maximum value of the random numbers. The resulting populations of framboids and sunflowers can be compared with data obtained from analysis of real samples in order to understand and model the genetic paleo-processes.