Banded Iron Formations (BIFs) are Precambrian sedimentary rocks interpreted to have been precipitated from anoxic seawater prior to the first permanent rise in atmospheric oxygen in the Great Oxidation Event at ca. 2.45–2.32 Ga. BIFs hold the key to understanding the chemistry of the oceans and atmosphere, and how these interacted with microbial life, prior to and during the evolution of oxygenic photosynthesis. To unlock this information, it is essential to know how BIFs formed, that is, what minerals were precipitated, by what mechanisms and in which environments. BIFs are found in almost all depositional settings at times when input of clastic detritus was lacking, for example, submarine proximal volcanic environments, basin floor, slope, deep marine shelf and shallow shelf settings. High-resolution electron microscopy of finely laminated BIFs and ferruginous cherts that range in age from 3.45 to ca. 2.4 Ga and that preserve depositional features indicates that the original sediment was a very fine mud composed of nanoparticles of the iron-silicate greenalite. In places, the greenalite mud experienced very early silicification on the sea floor enabling preservation of the mineral and its depositional textures. Very fine grained siderite and hematite in laminated BIFs post-date dehydration of the early silica cement and are not primary minerals. Greenalite nanoparticles are found in BIFs from all depositional settings indicating a common origin, likely precipitation resulting from mixing of plume water from hydrothermal vents with ambient seawater. The nanoparticles were carried throughout the oceans and were deposited on the seafloor and on continental margins to form the primary sediments of BIFs.