I'm just fascinated with these latest Pancam and MI pictures. What are these spherical pebbles and what is that bedrock that they seem to be pouring out of? Are they oolites? Lapilli? Spherulites? Amygdules?
I'm not a geologist so this is all completely amateur speculation but I'm starting to think that it's unlikely that they're oolites. Oolites would be interesting because they would signal tidally active bodies of water. Oolites form when a small particle accretes layers as it washes around in mineral-rich waters. Here on Earth, oolite rocks like oolitic limestone seem to form from densely packed beds or "sand bars" of these oolites. Looking at the MI pictures, it doesn't look at all like the dense packed oolites that I can find pictured and explained online. Oolites range in size from sugar-grain sized to several millimeters.
What about accretionary lapilli? Accretionary lapilli are like volcanic hailstones that form by the addition of concentric layers of moist ash around a central nucleus. They form in the ash clouds or columns from a violent volcanic eruption. On Earth, accretionary lapilli range in size from about 2 mm to as large as 6 or 7 cm. That matches up pretty well with the sizes seen in the Microscopic Imager where the granules are between 1 mm and 4 mm.
There's another kind of volcanic lapilli that are spheroid, teardrop, dumbbell, or button shaped. These granules are not accretionary, but are the direct result of liquid rock cooling as it travels through the air. This type of pyroclastic debris ranges in size from a few millimeters to several centimeters. On Mars, they would likely result from both volcanic activity and from meteor impact. I don't think it's likely that's what we're dealing with here because so many of these pebbles are so uniformly spherical. Could Mars' thinner atmosphere have sufficiently less drag on these liquid ejecta as to allow them to form more consistently into spheres? I don't know.
Or are these spheroids some kind of spherulites? Spherulites form mostly in glassy igneous rocks like obsidian (this bedrock looks anything but glassy to me). Some of the pictures I've found on the web do seem reasonably close, however, to what we're seeing at Meridiani.
What about amygdules? Amygdules form when the vesicular cavities (created by expanding gas bubbles in igneous rock) are filled with a secondary mineral. If this bedrock is a basalt, then amygdules seem like a distinct possibility to me. This picture of an amygdule here on Earth, looks like it could be a close match for that bedrock at Meridiani.
Now, I have to remind any of my geology knowledgeable readers that I'm not a geologist and I know very little about geology, so if I've suggested impossible hypotheses here, it's because I'm just making uneducated guesses. If you can help evaluate any of these possibilities, please share your knowledge (or guesses) with us in the comments. It really is curious and I suspect the real scientists will have some answers for us starting with tomorrow morning's press briefing. What a great mystery :)
