Formation process of pyrite polyframboid based on the heavy-metal analysis by micro-PIXE

Abstract

Pyrite framboid in sedimentary rocks could be concerned with arsenic contamination in groundwater of acidic environment and has been studied for the formation process of its unique morphology. However, little has been discussed about the formation process based on heavy-metal distribution in pyrite framboids and their aggregates. To reveal the formation process of pyrite framboids and their aggregates, mudstone from the Late Cretaceous Hakobuchi Group, central Hokkaido, Japan, are investigated for mode, petrographical, mineralogical, and micro-PIXE (particle-induced x-ray emission) analysis in this study. Spherical and sub-spherical pyrite framboids observed in polyframboids can be divided into two types based on diameter of framboids (D) and the average diameter of microcrystals (d) within framboids: type 1, ranging from 2 to 9 μm and from 0.4 to 0.9 μm, respectively, and type 2, ranging from 8 to 50 μm and from 0.5 to 1.8 μm, respectively. Based on the quantitative traverse analyses and 2-D elemental mapping results by micro-PIXE, heterogeneities in the concentrations of heavy metals within the analyzed aggregations of pyrite are exhibited. On the basis of the As and Pb zoning patterns, the time range and chronological stages of pyrite-aggregation growth are revealed.

Formation of Pyrite Spherules From Mixtures of Biogenic FeS and Organic Compounds During Experimental Diagenesis

“…12 of 16 Sawlowicz, 1993;Sugawara et al, 2013;Wei et al, 2012;Wilkin et al, 1996) but described using various names related to framboids, even though they differ from true framboids by their massive cores with undefined crystallites. The use of the term framboid to describe pyrites with significantly different textures, either formed in abiotic experiments (Butler & Rickard, 2000;Wolthers et al, 2005) or in the presence of bacterial biofilms (Large et al, 2001;Maclean et al, 2008;Vietti et al, 2015), complicates the discussion regarding their origin.…”

“…Alternative models for framboids formation involve the dissolution of FeS and subsequent pyrite precipitation from aqueous FeS reaction with 2 H S E without greigite intermediate step (Butler & Rickard, 2000). Many sedimentary pyrite spherules that do not fit the proper definition of framboids are designated by a plethora of names such as "proto-framboids" (Large et al, 2001), "packed framboids" (Sawlowicz, 1993), "welded-spheres" (Wilkin & Barnes, 1997b;Wilkin et al, 1996), "infilled framboids" (Böttcher & Lepland, 2000;Bryant et al, 2020;Wilkin & Arthur, 2001;Wilkin et al, 1996Wilkin et al, , 1997, "framboid ghosts" (Wei et al, 2012), or "united framboids" (Sugawara et al, 2013).…”