[S-CG45] Petrology, Mineralogy & Resource Geology

[SCG45-P14] Origine of goethite framboids in sandstone in the Saiki Subgroup of the Lower Shimanto Group, southern Kyushu, Japan

*Hiroaki Ohfuji¹, Seira Konishi¹, Keiji Yamamoto² (1.Tohoku University, 2.Kagoshima University)

Keywords:framboidal aggregate, goethite, microtexture, electron microscopy

The raspberry-like framboidal aggregate is a unique crystallization texture that can be frequently seen in microscopic pyrite. Framboidal pyrite occurs widely as a typical authigenic mineral in sedimentary rocks and modern sediments, and its size distribution has been used as an paleoindicator of the surrounding redox environment. On the other hand, framboidal aggregates composed of iron oxide/oxyhydroxide such as hematite and goethite, have also been reported, although they are relatively rare. They are considered to be pseudomorphs after framboidal pyrite, although their mineralogical characteristics have not been investigated in detail. In addition, some carbonaceous chondrites such as Tagish Lake meteorite contain framboidal magnetite, which is thought to have precipitated directly from fluid. It is interesting to see whether iron oxides can also form framboidal texture on the Earth's surface. In this study, micro- and nano-scale analyses were performed using SEM and TEM on iron oxide framboids, which were recently found by Egami et al. (2021) in sandstones of the Saiki Subgroup of the Lower Shimanto Group distributed in western Kagoshima, Japan, to understand their origin.
The results show that framboidal aggregate are distributed locally in the sandstone and often form clusters (polyframboids). Framboids are mostly surrounded by clay minerals (montmorillonite) produced by alteration, but no sulfur-containing minerals such as pyrite and sulfate minerals were observed. In the cross section of the framboidal aggregates, microcrystals are often infilled by secondary growth products, and Si and Al were also detected from the interstitial area between microcrystals by EDS analysis. In order to examine the microtexture of framboidal aggregates in more detail, thin films were cut out using FIB and observed by TEM.
TEM observation revealed that microcrystals are not single crystals, but are filled by assemblages of nano-sized crystals with a concentric layer pattern, which consists of several alternating layers of goethite nanocrystals and a mixture of clay minerals and amorphous silica. The goethite nanocrystals grew inward from the microcrystal outline, indicating that they were formed by alternation (replacement) of pyrite microcrystals. We also observed overgrowth of goethite nanocrystals growing outward from the periphery of the microcrystals, which are considered to be the main components of the infillings observed on the cross-section. Based on these observations, it is concluded that the present goethite framboids are pseudomorphs after pyrite framboids, which formed in the interstitial spaces in the formation during diagenesis.