...during a pre-dinner blood-sugar drop.

"Aragonite is deposited by hot springs; found associated with beds of gypsum and deposits of iron ore...found in amygdaloidal cavities in Basalt."

From Hurbut and Klein, Manual of Mineralogy, after Dana.

It is a higher pressure, higher temperature phase. Linear relationship. Lower the temp and you lower the pressure of the stability field. The graph in Hurlbut and Klein has an origin of 0 temp and 0 kilobars, but if one projects the line, when it gets really cold the pressure drops very low as well. Or vice-versa, pressure-wise. In addition, it often forms crystals first, then is replaced (psuedomorphed) by a calcite phase.

From Hurlbutt and Klein:

So, at a very low pressure, you don't need all that hot a hydrothermal system to deposit aragonite. A warm spring will do. Also, hydrothermal sytems are not a single temperature. As they get closer to the surface, many things happen. Temperature drops, pressure drops, it boils, all kinds of things happen to the physical characteristics of the fluid, and thus in the kinds of minerals they deposit. (that's what makes many economic mineral deposits!)

My prefered model for these weird, white, sedimentary looking rocks* Spirit has been seeing lately: they are alteration products at the top of basalt flows between flow events. There could have been low-fluid volume hydrothermal cells developed within the cooling basaltic flow, which eventually cools completely, runs out of water, or is covered by another basalt flow. Or, they are low temp Sabkha-esque deposits, at a much smaller scale than the "Opportunity Formation" sulfates.

This rock has a texture that may be a relict of its unaltered precurser rock - basalt.

Another spectra I hope we see soon.

*different than Wishstone!