Metamorphic Formation of Extraterrestrial Portlandite in the Sutter's Mill Meteorite (SM3)

The Sutter's Mill meteorite fell on April 22nd, 2012. Only three small stones (totaling 14.6 g) were collected before heavy rains fell over the fall site, one of which (SM3, 5.0 g) was obtained by Arizona State University's Center for Meteorite Studies. Bulk powder X-ray diffraction (XRD) investigation of seven stones shows that SM3, 6, 8 and 9 are olivine-rich and SM38, 41 and 65 are clay-rich [Garvie 2013]. The olivine-rich stones are largely anhydrous, with mass losses of ~3 wt%, as measured by thermogravimetric analysis (TGA). SM3 also contains Fe-sulfides, magnetite, oldhamite, and minor enstatite. Reflected-light observations show a heterogeneous distribution of clasts, chondrules, sulfides and bluish-white grains embedded in a dark, fine-grained matrix. Three visually prominent bluish-white mineral grains were identified for study: Grain 1, 100 um surrounded by matrix; Grain 2, 200 x 100 um with a rim of ferrous olivine; and Grain 3, 350 x 150 um surrounded by a thick rim of microcrystalline Fe-Ni sulfides. Wavelength dispersive spectrometry (WDS) data of these grains are dominated by Ca and O exhibiting a 1:2 Ca:O ratio, with minor Cl and S. Secondary ion mass spectroscopy (SIMS) reveals abundant H. Compositional maps show an even distribution of Ca across the grains, with enrichments of S at the rims. The chemical data of these grains is consistent with portlandite, Ca(OH)₂. This is the first indigenous report of meteoritic portlandite. Portlandite can form through the thermal decomposition of CaCO₃ or via the carbothermic reduction of CaSO₄ to CaO. CaCO₃ decomposes to CO₂ and CaO at temperatures >840° C. Carbothermic reduction of CaSO₄ to CaO can occur at temperatures >700° C. Both reactions produce CaO which can then easily hydrate to Ca(OH)₂, with a likely source of H from dehydroxylation of pre-existing serpentines. Dehydroxylation of serpentine occurs between 550° and 800° C with complete dehydration to olivine >800° C [Ivanova 2010, Gualtieri 2012]. Given the anhydrous nature of SM3, generation of Ca(OH)₂ through the thermal decomposition of CaCO₃ is unlikely as the dehydroxylation of serpentine would be complete and a source of H would be absent. A more likely mechanism for the formation of Ca(OH)₂ in SM3 is the carbothermic reduction of precursor CaSO₄ using CO and CO₂ evolved from carbon, which is present within C-type chondrites. These data suggest that SM3 experienced temperatures as high as 700° C. Understanding the formation of Ca(OH)₂ provides new insights into thermal processing of carbonaceous chondrites.