Rusticle

A rusticle is a formation of rust similar to an icicle or stalactite in appearance that occurs underwater when wrought iron oxidizes. They may be familiar from underwater photographs of shipwrecks.

The rusticle consists of up to 35% iron compounds including iron oxides, iron carbonates and iron hydroxides. The remainder of the structure is actually a complex community of symbiotic or mutualistic microbes including bacteria and fungi that use the rusting metal as a source of food, causing microbial corrosion and collectively producing the mineral compounds as waste products and hence forming the rusticle.

Since rusticles are found on wrought iron rather than other ferrous metals, it is supposed that the microbes also use the sulfur and phosphorus impurities found in the metal.

Structurally, rusticles contain channels to allow water to flow through, and they seem to be built up in a ring structure similar to the growth rings of a tree. They are very delicate and can easily disintegrate into fine powder on even the slightest touch.

The outer surface of a rusticle is smooth red in appearance from the iron (III) oxide, while the core is bright orange due to the presence of Goethite (a hydrated iron oxide) crystals.

The word "rusticle" is a portmanteau of the words "rust" and "icicle" and was coined by Robert Ballard, who first observed them on the wreck of the "Titanic" in 1986. Rusticles on the "Titanic" were the first investigated in 1996 by Roy Cullimore, based at the University of Regina in Canada.