Hematite concretions known from Earth (e.g. Utah, Fig. 1a) usually form by subsurface precipitation in flowing water. Marble-shaped pebbles ("Blueberries", Fig. 1b) were also discovered on Mars by the Mars Exploration Rover Opportunity.
Hematite (Fe2O3) is a mineral that in most cases requires water for its formation, but it can also result form a dry, thermal oxidation process. For Meridiani Planum a formation by water is assumed [Hynek et al., 2002]. The presence of water in turn is linked to life. Therefore hematite indicates wetter and warmer conditions on Mars in time of hematite formation.
There are many explanations of how the hematite in Meridiani was formed [Chan et al., 2004]:
On Mars hematite appears as spherules on surface layers and in shallow depressions (Fig. 2). The "grain" size is comparable to small blueberries (< 0.5 cm) [Chan et al., 2004] and the iron source is supposed basaltic (> 80 % [Christensen et al., 2001] or andesitic [Bandfield et al., 2000]. The Martian hematite is a grey crystalline hematite [Christensen et al., 2000a; Christensen et al., 2000b; Christensen et al., 2001; Hynek et al., 2002; Catling and Moore, 2003; Christensen and Ruff, 2004; Ormö et al., 2004; Glotch et al., 2006], a pure hematite having a larger crystal structure than the reddish iron oxide known as rust. It formed as a secondary hematite out of Fe-rich strata in an acidic groundwater influenced environment [Hynek et al., 2002].
On Earth they are also accumulated on surface layers and in topographic lows as spherules or other shapes (depending on the host texture) on the scale of 1-20 cm in diameter. In Utah the host rock is a sandstone (fine- to medium-grained quartz arenite) cemented by hematite making up a few percent of the rock [Chan et al. 2000]. The iron source is hematite grain coatings. Hydrocarbon-rich, brine-influenced fluids were mixed with oxygen-rich groundwater circulating through this permeable rock. Chemical reactions then made the minerals precipitate to form layered spherical pebbles. The surrounding Jurassic Navajo sandstone was eroded with time and the hard resistant concretions accumulated on the ground (Fig. 3).
Last update: 07/06/2010 14:41