A mechanism for framboid formation as illustrated by a volcanic exhalative sediment

Abstract

A sequence of oxidate facies sediments associated with a cupriferous massive sulphide deposit in the Solomon Islands is the product of precipitation at an exhalative fluid/seawater interface. A siliceous sinter bed within the sediments contains magnesioferrite spheroids having a distinctive framboid texture. Scanning electron micrographs of the spheroids show an internal structure of parallel rods composed of sub-spherical microcrysts. The formation of the magnesioferrite framboid texture is ascribed to the coagulation of magnetic iron hydroxide gel particles due to magnetic attraction but facilitated by the presence of a strong electrolyte. By analogy a simple mechanism to account for pyrite framboid formation is proposed. This mechanism requires primary iron sulphide particels to be attracted to one another because of the ferrimagnetic properties of a precursor FeS polymorph or alternatively by Van der Waals forces accentuated by the presence of charged ions in a strong electrolyte. Ordering of resultant microcrysts is mainly a close-packing effect which produces robust aggregates resistant to deformation during subsequent diagenesis.