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

A Morphological and Size-Based Study of the Changes of Iron Sulfides in the Caples and Torlesse Terranes (Otago Schist, New Zealand) during Prograde Metamorphic Evolution

“…This REV depends on the occurrence and size of the measured property. For MPy, previous works [12][13][14] have concluded that the REV is achieved when the number of objects is above 600-1000. The size and distribution of the MPy can be adjusted to a log-normal distribution, according to the Crystal Size Distribution (CSD) and the law of proportionate effect [28], which explains how a crystal population tends to grow in proportion to its size.…”

“…used to segregate the MPy subpopulations. More details about this process can be found in [11][12][13][14]. According to [27], an average sample volume scanned by MCT is much larger than the representative elementary volume (REV).…”

“…However, there are two issues related to this technique: the representativeness of the volume scanned and the underestimation of the true diameter, as it involves measuring a 3D object via a 2D surface. Recently, high-resolution X-ray computed tomography (MCT) [10], together with statistical analysis based on Gaussian finite mixture models, have been used to characterize the size and abundance of iron sulfide populations [11][12][13][14]. This method allows the populations of iron sulfides to be measured in 3D, in a fast and accurate way, preventing possible errors due to the 2D observation of sections as performed in SEM studies.…”

Ooidal ironstones in the Meso-Cenozoic sequences in western Siberia: assessment of formation processes and relationship with regional and global earth processes

“…3d; cf. Merinero et al 2009Merinero et al , 2017Merinero and Cárdenes 2018). The diameter of this rare variety of framboids varies from 6 μm to 20 μm.…”

“…The occurrence of small variety of pyrite framboids in the three intervals in well 570 indicates oxygendepleted depositional conditions (cf. Berner 1984;Wilkin et al 1996;Wei et al 2015;Merinero et al 2017;Rickard 2019). The abundance and size of framboids indicate their formation in the hydrogen sulfide-rich bottom water column.…”