St. Catharines 2004
Technical Program

SS19:  Molecules to Planets: Infrared spectroscopy in geochemistry, exploration geochemistry and remote sensing 
Organizers / Organisateurs:  Penny King, Mike Ramsay, Gregg Swayze
Room / Salle:  AS 201

Date:  05/12/2004
Time:  10:00 AM
Presenter:  Michael D. Kraft

Thermal emission spectra of secondary silicates formed in arid-zone basalt weathering

Kraft, M.D., Michalski, J.R., and Sharp, T.G. Department of Geological Sciences, Arizona State University, Tempe, AZ, USA, 85287-1404,

Thermal emission remote sensing measurements of natural surfaces reflect a combination of rock minerals, weathering rind minerals, and soils, which may contain both primary and secondary minerals. Secondary minerals can obfuscate quantitative mineralogical analyses of rocks. But secondary minerals, as measured in thermal emission, also can reveal important information about weathering processes and the extent of alteration of natural surfaces.

We are concerned with secondary silicates that develop on mafic rocks under arid conditions. These include clay minerals (smectites and halloysite) and amorphous or poorly crystalline authigenic silicates (opaline silica and allophane). Clay minerals commonly develop from weathering of basaltic rocks and comprise a significant fraction of mafic soils. Clays may also form a fraction of authigenic silicates in weathering rinds. Poorly crystalline silicates are important constituents of basalt weathering rinds and they are known to form high-silica rock coatings in arid regions. Silicate mineraloids form in mafic soils; allophane, in particular, can make up a significant fraction of soil silicate materials.

As a first step in addressing thermal emission measurements of natural basaltic surfaces, we have performed a detailed thermal emission spectroscopic study of secondary silicate minerals, including characterization of spectra in a crystal chemical sense. We have collected spectra of silicate weathering products including varieties of clay minerals, opaline silica, allophanes, and silica-clay physical mixtures. The positions of absorption features in the 900-1300 cm-1 range resulting from Si-O stretching are a function of silicon-oxygen ratios, with high Si:O material absorption minima positioned at higher energy. As such, the absorption minimum for pure SiO2 opal occurs at ~1115 cm-1; the minima for clays (Si:O = 0.3-0.4) occur in a lower wavenumber region, 1030-1075 cm-1. Absorption minima for secondary amorphous silicates with Si:O between 0.4 and 0.5, such as aluminous opal and some allophanes, are positioned between absorption minima of opal and clays. As a happenstance of their amorphous nature and Si:O ratios, these materials are spectrally similar to high-silica glasses, such as obsidian.

This study has import bearing on interpretations of thermal infrared data for natural, weathered basaltic surfaces on Earth and for thermal emission remote sensing of Mars. For the Martian case, a global unit with a strong absorption measured at ~1085 cm-1, previously attributed to obsidian-like glass, may result from weathered basaltic surfaces containing aluminous opal or allophonic material.