J. Ormo had a poster at the LPSC meeting which was, perhaps, the single most interesting presentation at the meeting: his team has routinely created round Blueberry-type concretions without the need for any kind of different chemical nucleus for them to begin crystallizing around -- including any bits of organic matter, which have been proposed as nuclei for them in the past. (http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1356.pdf)
They mixed ferrous sulfate and potassium hydroxide solutions within a sand matrix:
"In the first experiment, the sand was soaked with FeSO4. Then KOH was dropped (about 2 drops per second, 400μl total) on one spot of the soaked sand. In the second experiment, the same volume of FeSO4 solution was dropped on sand soaked with KOH solution. In a third experiment, the sand in the central part of the dish was soaked with KOH solution and, simultaneously, the sand in the periphery with FeSO4 solution. This caused the two fluids to meet with reaction only at the front of the contact.
"Results: Immediate interaction of the injected fluid with the soaked substrate forms a first, greenish precipitate at the chemical interface. The injected fluid continues to spread until, suddenly, a more distinct, soft, greenish rind of iron hydroxide forms. This halts further spread of the fluid. The rind of the 'concretion' then grows inwards independently if the experiment is done with the iron-solution mainly on the outside or the inside of the rind. The rind growth stops after a few hours, possibly because the rind gets hermetically sealed." [The experiments were then elaborated.]
"Conclusions: Field observations and laboratory experiments indicate that spherical iron oxide concretions can form without growth from a nucleus. Instead, self-organized zones of super-saturated (above nucleation threshold) solution and diffusion cause spherical precipitates of iron-hydroxide. Chemical gradients between the inside and the outside of these spheres cause diffusion of Fe ions towards the outer perimeter of the amorphous sphere forming a rind. The rind then grows inwards due to diffusion within the sphere. Breaches in the rind may cause 'onion layering'. Massive concretions may form when the rind growth can proceed towards the center. Dehydration and/or an increase of temperature change the iron-hydroxide to iron-oxide. The occurrence of specular gray hematite in the Meridiani spheres suggest some unique conditions of diffusion with supersaturated solutions and temperature/pressure regimes that favored the formation of hematite (with or without intermediate iron oxide stages), as well as an abundant and reactive iron source."
As they say, this isn't the specific type of chemical reaction thought to have occurred at Meridiani -- but it's similar enough to indicate that the Meridiani Blueberries did not necessarily need any kind of a chemical nucleus, organic or otherwise. As such, it may in itself solve perhaps the single most important remaining mystery about Meridiani.
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