Icosahedral domain structure in framboidal pyrite - An example of extraordinary self-organization in micrometer scale -(<Special issue>Mineralogy-Micro to Global- (Part2))

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  • フランボイダルパイライトの20面体ドメイン構造 ―ミクロスケールにおける粒子の特異な自己組織化の一例―(<特集>鉱物学,ミクロからグローバルを探る(その2))
  • フランボイダルパイライトの20面体ドメイン構造--ミクロスケールにおける粒子の特異な自己組織化の一例
  • フランボイダルパイライト ノ 20メンタイ ドメイン コウゾウ ミクロスケール ニ オケル リュウシ ノ トクイ ナ ジコ ソシキカ ノ イチレイ

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Abstract

Icosahedral architecture is extremely rare in natural materials and prohibited in minerals (bulk crystals) because it involves fivefold axes which are not compatible with translation symmetry. Here, we review an interesting icosahedral domain structure found recently in natural pyrite framboids, which are microscopic spheroidal aggregates of equant, equidimensional pyrite microcrystals and are ubiquitous in a variety of geological environments. The icosahedral structure in framboids consists of twenty tetrahedral subunits in which the internal microcrystals are regularly arranged in a cubic close packing and essentially involves fivefold and threefold microcrystal distributions on the sections. This is the only close packed icosahedral assembly of micrometric particles found in the natural world. Similar icosahedral structure based on the poly-tetrahedral construction can be observed in some synthetic nanoparticles (nanoclusters) such as multiple twinned particles of Au, Pd, etc. and chemical vapor deposited (CVD) diamonds. However, there are large differences between these icosahedral nanoparticles and framboids in terms of the attributes of the constituent particles. For example, the former exist at atomic dimensions, whereas the latter are composed of single crystals which are 10^4 times larger than atoms. Therefore, investigating such a micrometer scale icosahedral self-assembly might play an important role in understanding the physical state and behavior of small particles from nanometer to micrometer scales and, potentially, also to macro scale structure.

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