RusticleA Rusticle is a little similar to an icicle or stalactite in appearance, but occurs under water when wrought iron rusts. 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, collectively producing the mineral compounds as waste products and hence forming the rusticle.
Structurally, the rusticle contains 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 easily fall apart into fine powder if knocked.