Introduction

Framboids are microscopic subspherical clusters of equant and equidimensional microcrystals. They overwhelmingly consist of the mineral pyrite, cubic FeS₂. There are about 10³⁰ framboids on Earth and they are forming at a rate of about 10¹⁴ per second. They may be the most abundant mineral texture on Earth. Framboids are especially concentrated in sediments, although they are also to be found in the water column and in high temperature systems. The oldest framboids are possibly 2.9 Ga and they are found in all geologic periods from that time. The first framboids were described in 1885 from a peat bog, and the term framboid was coined in 1935. They have fascinated researchers ever since, not least because a substantial fraction of them display astonishing regular microarchitectures where their constituent microcrystals are geometrically ordered. Understanding of the nature of framboids has paralleled technological advances in microscopy, structural and chemical analyses, and computing. The sulfur in sedimentary framboids is almost exclusively sourced from sulfate-reducing bacteria, and the idea that framboids were fossil microorganisms was first propounded in 1923. Subsequently, the limited distribution of organic matter in framboids, its absence in hydrothermal framboids, and inorganic framboid syntheses showed that organisms were not necessary for framboid formation.