Phyllosilicates (micas, clay minerals) occur in the Earth crust besides quartz and feldspar. Often they are secondary alteration products of feldspars etc. Clay minerals are important in alteration, pedogenesis, and diagenesis. On Earth, high occurrences of marine, mica- and glauconitic sandstones, marls, shales, and limestones are common in flysch (The Alps: Lower Cretaceous - Upper Eocene).
Phyllosilicates detected on Mars (Fig. 1) are mainly associated with Noachian outcrops indicating an early active hydrological system. In Nili Fossae mafic minerals (olivine, pyroxene) and phyllosilicates (clay minerals) have been identified with OMEGA at different locations. Three locations only show phyllosilicates on smooth layered deposits, massive exhumed material and crater ejecta and one shows phyllosilicate and olivine signatures on an altered mafic unit [Mangold et al. 2007]. The Noachian basement exhibits strong exposures of LCP (low calcium pyroxene) - mixed with HCP (high calcium pyroxene) decoupled from hydrated minerals which means that large parts of the crust are not hydrated.
The distribution of hydrated minerals indicates a widespread liquid water activity and various alteration processes. As hydrated minerals are found exclusively on the Noachian basement their formation processes were active during the Noachian but inactive after the Syrtis Major lava flow emplacement (Hesperian) lacking hydrous minerals.
These phyllosilicates were identified as clay minerals. Clay minerals generate pseudomorph from primary existing silicate layer structures particularly out of di-octahedral- (muscovite) and tri-octahedral micas (biotite) during alteration [Matthes, 2001]. There the potassium ion (K+) is discharged partially or completely, and then layer charge is accordingly decreased. Both mica structures may form illites (incomplete micas) containing lower potassium ions and a higher Si-Al-ratio than muscovite. Some alteration processes form vermiculite, which is a swellable phyllosilicate. In many cases mixed layer minerals between illite and montmorillonite up to pure swellable montmorillonite are formed. Montmorillonite is the major component of the clay mineral bentonite that forms by alteration of volcanic ash (basic volcanics).
Last update: 20/05/2010 14:15
