R2: Crystal structure, crystal chemistry, physical properties of minerals, crystal growth and applied mineralogy

[R2-06] Origine of framboidal goethite in sandstone in the Saiki Subgroup of the Lower Shimanto Group

*Hiroaki OHFUJI¹, Seira Konishi¹, Hiroshi Yamamoto² (1. Tohoku Univ. Sci, 2. Kagoshima Univ. Sci)

Keywords:Framboida goethite, Framboidal pyrite, Clay mineral, Alteration

The raspberry-like framboidal aggregate is a unique crystallization texture that can be frequently seen in microscopic pyrite in sedimentary rocks and modern sediments. In this study, we conducted micro- and nano-scale analyses 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 and/or infilled textures. Framboids are mostly surrounded by clay minerals produced by hydrothermal alteration, but no sulfur-containing minerals such as pyrite and sulfate minerals were observed. TEM observation on thin foils prepared by FIB showed that microcrystals that constitute framboids 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. These observations suggest goethite framboids are pseudomorphs after pyrite framboids, which formed in the interstitial spaces in the formation during diagenesis.