Quartz in banded iron formations (BIFs)

The thickness of the iron-bands and quarz-bands depends on the time how long either the iron-rich dark sediments or the quarz-sediments were deposited. There was some rhythmic change of sedimentation depending on the changing quality of the sea-water.

The time for the crystallisation depends on the concentration of the substance in the solution, the purity of the substance, the temperature and how quiet the water is, in which the crystallisation takes place.

http://innovative-planetary-science.page.tl/Earth-Formation.htm

http://en.wikipedia.org/wiki/Banded_iron_formation