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Powder X-ray diffraction shows that K- and Ca-exchanged montmorillonites swell upon interacting with CO(2) at ambient temperatures, depending on their initial hydration state. K-exchanged montmorillonite swells rapidly to a maximum d(001) of ∼12.2 Å. In contrast, Ca-exchanged montmorillonite swells more slowly, but reaches a maximum d(001) of ∼15.1 Å. Reaction kinetics differ significantly between the K- and Ca-exchanged montmorillonite complexes. Expansion of K-exchanged montmorillonite samples was rapid, occurring on time scales of tens of minutes or less. The Ca-exchanged montmorillonite samples continued to expand over periods up to 42 h. Aging of both K- and Ca-exchanged montmorillonite complexes at elevated CO(2) pressure for 1-2 days resulted in greater stability when CO(2) pressure was released. The observed intercalation reactions have important consequences for carbon sequestration: (1) CO(2) absorption by swelling clays may represent a significant pathway for storage of CO(2). (2) The swelling of smectites under CO(2) pressure may have a significant impact on the permeability of caprock formations.

Amphibole in chassignite melt inclusions provides valuable information about the volatile content of the original interstitial magma, but also shock and postshock processes. We have analyzed amphibole and other phases from NWA 2737 melt inclusions, and we evaluate these data along with published values to constrain the crystallization Cl and H 2 O content of phases in chassignite melt inclusions and the effects of shock on these amphibole grains. Using a model for the Cl/OH exchange between amphibole and melt, we estimate primary crystallization OH contents of chassignite amphiboles. SIMS analysis shows that amphibole from NWA 2737 currently has 0.15 wt% H 2 O. It has lost~0.6 wt% H 2 O from an initial 0.7-0.8 wt% H 2 O due to intense shock. Chassigny amphibole had on average 0.3-0.4 wt% H 2 O and suffered little net loss of H 2 O due to shock. NWA 2737 amphibole has dD % +3700&; it absorbed Martian atmosphere-derived heavy H in the aftermath of shock. Chassigny amphibole, with dD ≤ +1900&, incorporated less heavy H. Low H 2 O/Cl ratios are inferred for the primitive chassignite magma, which had significant effects on melting and crystallization. Volatiles released by the degassing of Martian magma were more Cl-rich than on Earth, resulting in the high Cl content of Martian surface materials.

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Potassic-chloro-hastingsite has been found in melt inclusions in MIL 03346, its paired stones, and NWA 5790. It is some of the most chlorine-rich amphibole ever analyzed. In this article, we evaluate what crystal chemistry, terrestrial analogs, and experiments have shown about how chlorine-dominant amphibole (chloro-amphibole) forms and apply these insights to the nakhlites. Chloro-amphibole is rare, with about a dozen identified localities on Earth. It is always rich in potassium and iron and poor in titanium. In terrestrial settings, its presence has been interpreted to result from medium to high-grade alteration (>400°C) of a protolith by an alkali and/or iron chloride-rich aqueous fluid. Ferrous chloride fluids exsolved from mafic magmas can cause such alteration, as can crustal fluids that have reacted with rock and lost H 2 O in preference to chloride, resulting in concentrated alkali chloride fluids. In the case of the nakhlites, an aqueous alkali-ferrous chloride fluid was exsolved from the parental melt as it crystallized. This aqueous chloride fluid itself likely unmixed into chloride-dominant and water-dominant fluids. Chloridedominant fluid was trapped in some melt inclusions and reacted with the silicate contents of the inclusion to form potassic-chloro-hastingsite.