Southeastern Section–55th Annual Meeting (23–24 March 2006)

Paper No. 9
Presentation Time: 11:00 AM


FOLK, Robert L. and CARLIN, Joseph P., Department of Geological Sciences, Univ of Texas, Austin, Austin, TX 78712,

An iron birdbath rusts. How is the rust being precipitated? Conventional wisdom would say that is may be in part simply an inorganic oxidation process, and in part driven by iron bacteria such as Leptothrix. But what is the nanomorphology of the product? Before we try to interpret Martian iron oxides we need to understand them on Earth.

Fallen leaves lie in the birdbath water and become coated with fluffy, pumpkin-orange iron oxide, Munsell 5YR 5-7/6-8. This forms clumps (? colonies) and sheets of 30-100 nm balls. When the leaf is placed in 10% HCL for one week, the iron oxide is removed but the balls remain insoluble, thus we conclude that they are acid-resistant cells of nannobacteria that precipitated the iron oxide.

At the iron/water contact on the bottom of the pool, brittle crusts of iron oxide form, about the color of a dark chocolate bar, 1OR 2/2. This is strongly magnetic and shows a wide variety of morphologies. Most common are “euhedral” hexagonal plates 1 ìm thick by 2-12 ìm wide; but these are made of stacked sheets about 30 nm wide that are studded with 30 nm balls and ovoids. In other “crystals” the balls range 50-150 nm. The plates have the morphology of hematite, but the strong magnetism suggests delta-FeOOH. Also there are fields of 3 ìm rosettes; 30-70 nm balls join into straight chains, then into sheets, finally into “flowers” resembling carnations.

The balls that make up the “crystals” and “carnations” are interpreted as nannobacterial cells, much the same size and shape as those that form other minerals, harden human arteries, and are found in Martian meteorites. In the iron birdbath the iron oxides are (nanno)bacteriogenic.