In this project, emphasis is placed on characterization of the interfaces between primary and secondary minerals. Note that these images were collected using a transmission electron microscope and that this instrument, which is capable of atomic resolution, provides structural and chemical measurements with nanometer-scale resolution.

The resolution of atomic level structural details is illustrated by our work on intergrown layer silicates. Here we see black spots that correspond to projected pairs of silica tetrahedra, dark stripes corrsponding with brucite-like interlayers, and grouping of these representing 1:1 serpentine layers, 2:1 layers, and in combination with the brucite-like interlayers, clorite layers.

Here are some examples of results showing partially weathered silicate minerals:

(i) Amphibole (bottom left and central islands) is converted to smectite. Arrows show the nanometer-scale channels between the amphibole I-beams, presumably zones where fluid transport into and away from the reaction site occurs.

(ii) Pyroxene and smectite. Note the intimate contact between pyroxene (right hand side) and smectite (near-horizontal lattice fringes on righ hand side. As in the cases of amphibole, micas, and olivine, clay weathering products adopt an orientation dictated by the structure of the primary mineral.

weathered chlorite. Note that this image shows atomic resolution down the chlorite b axis. The darkest stripes are brucite-like interlayers. (B). Those that are not so dark have been altered by loss of cations! See Banfield and Murakami, Am. Min. in press.

(iii) The secondary minerals commonly formed as products of weathering of biotite include vermiculite and kaolinite. Note that this reaction involves oxidation of ferrous iron and removal of interlayer K (via an layer-by-layer mechanism).

(iv) Olivine also alters to topotactically oriented smectite in the early stages of weathering. Iron oxides are also common products in more highly weathered samples.

Ferromagnesian silicates remain crystalline to within a fraction of a nanometer of their surfaces, reaction products develop in highly specific orientations, and there is little evidence for the presence of a bulk fluid.

Through examination of exsolved minerals it is possible to evaluate the role of compositional variations in modifying silicate mineral reactivity (all other factors, such as climate, organic acids, etc. remain constant). Data suggest that compositional variation has little impact on weathering rates of some pyroxenes and amphiboles. However, in the case of exsolved alkali feldspars, evidence demonstrates very rapid rates of conversion of albite to clay relative to k-feldspar. Note that clay minerals are rich in elements released from adjacent weathering of pyroxenes.