QUOTE (dburt @ May 5 2008, 12:38 AM)
I hope this explanation helps.
-- HDP Don
Thanks! That helped muchly!
I guess the more basic question (sorry again!) is why martian basalts are necessarily of high pH? (more basic)
Please correct me if I'm wrong, but I'm assuming it is the metal oxides [CaO, MgO, and Na2O, Fe, Al and their buddies] in basalt, that on dissolution/exposure to water form metal hydroxides and suck up protons from the (minimally buffered) aqueous solution. By definition: lower proton concentration = lower pH.
A fairly recent abstract [Hurowitz et al. LPSC (2005) Abstract 2025. Experimental basalt alteration at low pH: implication for weathering relationships on Mars.
) mentions that the weathering relationships on Mars under low pH (high acid) conditions cause a different set of alterations. According to the abstract. At "neutral pH" (pH 5-8) alkali metals (Na, K, Ca, Mg) are leached out and leave behind Fe(III) and Al. At acid pH (pH 1-4), oxidation to Fe(III) is inhibited and Al is much more soluble than at neutral pH.
[Al(OH)3 is amophoteric - it dissolves better in basic or acid media than in neutral media due to the formation of different species at either acid or basic pH. In the chemical laboratory, aquous aluminum salt workups can make some nasty emulsions. (Hint: acidify your AlCl3 workups with with phosphoric acid)]
[More trivia: iron salts are less available to plants at neutral or basic pH. Certain plants that lack efficient siderophores (iron-chelating enzymes responsible for fetching this nutrient to cells), like blueberries, need a more acid soil to be able to thrive. This is why most blueberry plants do better at soil pH's in the range 4.5 - 4.8.]
The rock weathering pattern is pH dependent. Earth rock weathering patterns are generally consistent with neutral pH weathering. According to the abstract, the Mars surface rocks seem consistent with low pH surface weathering. By comparing the interior of Adirondack (and Humphery, too) rock at Gusev with it's (weathered) exterior, it appears as if olivine has been "dissolved" away from the outer layer of Adirondack.
Their concluding sentence: "Martian rocks and
soils do not mimic the weathering trends produced by
basalt alteration on Earth, indicating they have not
been altered by interaction with large volumes of moderate
pH rainfall or groundwater."
So it appears that any surface and near surface waters (if they were present) and the groundwater traveled in different circles.
Don't use well water for your martian blueberry farm