presentation (pdf) - University of Glasgow

High-resolution characterization of naturally
weathered mineral surfaces by FIB & TEM
David Brown, Maureen MacKenzie & Martin Lee
University of Glasgow, UK
Caroline Smith
Natural History Museum, London, UK
Mark Hodson
University of Reading, UK
Roland Hellmann
LGIT, Grenoble, France

Models for feldspar weathering

Models for feldspar weathering

Models for feldspar weathering

Models for feldspar weathering

These models are difficult to test because the most reactive
sites on the surfaces of mineral grains within soils will be:
• At the bottom of etch pits, and/or
• Beneath reaction products or organic/inorganic debris.

Our approach: 100 nm thick cross-sections of grain surfaces
were made using a focused ion beam (FIB) microscope for
imaging & chemical analysis by transmission electron
microscopy (TEM).
Alkali feldspars from two different soils were studied:
• Shap (north-west England)
• Acidic (pH 3.4) & waterlogged peat. Grain surfaces free
of weathering products.
• Glen Feshie (Cairngorm, Scotland)
• Soil chronosequence (80-10 kyr). Grain surfaces have
abundant weathering products.

FIB cross-sectioning
Using a 30 kV Ga + ion beam pairs of trenches are cut into grain
surfaces leaving an electron-transparent (~100 nm thick) slice:

FIB cross-sectioning
Using a 30 kV Ga + ion beam pairs of trenches are cut into grain
surfaces leaving an electron-transparent (~100 nm thick) slice:

FIB milling artifacts
Grain surfaces coated with

FIB milling artifacts
Modeling show that amorphisation develops in response to
damage associated with implantation of energetic Ga + ions:

Shap alkali feldspars: Microtopography & reactive sites
Grain surfaces are free of reaction products but pitted.
Corrugations form at the outcrops of coherent albite lamellae:

Shap alkali feldspars: Reactive sites
Albite dissolves more rapidly than orthoclase due to differences
in composition or the magnitude of elastic coherency strain:

Shap alkali feldspars: Etch pit networks
Cross-sectioning of weathered surfaces enables study of the
interconnectivity of pits in the grain interior:

Shap alkali feldspars: Etch pit networks
Pits extend into grain interiors as fluids exploit nanotunnels
after dislocations and dissolve their elastically strained walls:

Shap alkali feldspars: Microstructure of pit walls
Cross-sections of heavily weathered grains show how the pits
extend along former albite lamellae into the grain interior:

Shap alkali feldspars: Microstructure of pit walls
Etch tube walls are crystalline throughout. Amorphous ‘leached’
layers are entirely absent:

Glen Feshie alkali feldspars
Pits were dig into soils formed on river terraces of various ages:

Glen Feshie alkali feldspars
Most grain surfaces are partly covered by organic material
(fungal hyphae & diatoms) and weathering products:

Glen Feshie feldspars: Coated grain surfaces
Cross-sections reveal clearly the relationships between organic
material and weathering products on grain surafaces:

Glen Feshie feldspars: Weathering products
Some weathering products are amorphous, others crystalline
Fe-K aluminosilicates:

Glen Feshie (80 yr soil): Clay-feldspar interface
Poorly crystalline material occurs between clays and the grain
surface but there is little evidence for feldspar recrystallization:

Glen Feshie (1.1 kyr soil): Reconstruction at grain surfaces
Amorphous reaction products may have a gradational interface
with the feldspar, suggesting some interaction:

Glen Feshie (1.1 kyr soil): Crystallization of clays
Elsewhere the amorphous reaction products have crystallized
K-aluminosilicates:

Conclusions
The FIB-TEM technique enable detailed characterisation of
naturally weathered mineral surfaces in cross-section.
Shap grains weather by stoichiometric
dissolution centered on sites reactive by
virtue of their composition and/or strain.
Any ‘leached layers’ must be