presentation (pdf)

Nanoscale characterization of the ‘critical zone’ of
naturally weathered minerals by FIB and TEM
David Brown, Mark Hodson, Maureen MacKenzie,
Roland Hellmann, Caroline Smith & Martin Lee

Outline
• The critical zone: models for feldspar weathering
• Materials
• Focused Ion Beam (FIB) technique
• Results
• Integration with XPS
Aim
• FIB-TEM investigation of mineral weathering:
– Grain surface topography; structure; composition;
reaction products
– How do feldspars weather in natural systems?

Models for feldspar weathering

Models for feldspar weathering

Models are difficult to test:
• Reactive sites at the bottom of etch pits
• Beneath reaction products or organic/inorganic debris.

Advantages of FIB
• Previous techniques for TEM foil preparation destructive:
– (e.g. microtome, Ar ion milling)
• FIB allows site specific cross sectional slices to be
prepared, through the critical zone
• Direct observation of layers?

Materials
• Naturally weathered alkali feldspars from post-glacial
soils
• Shap (north-west England)
– Acidic (pH 3.4) & waterlogged peat.
Grain surfaces free of weathering
products.
• Glen Feshie (Cairngorm, Scotland)
– Soil chronosequence (80 yr-1 kyr).
Grain surfaces have abundant
weathering products.

FIB Technique

Beam Damage
• Sample can be damaged by
the beam
– 1 µm Pt strip
• Au coating essential
– Amorphisation of the
sample
• >85 nm of Au required
• Supported by SRIM
modelling (

Amorphous layers?
• No amorphous layers recognised on pH1 laboratory
dissolution experiments and on naturally weathered
samples
• Previous experiments – 10’s of nm, where are the layers?

• Can Au coating
(by sputtering)
damage sample?
• Grains partially
covered by resin
• No amorphous
material
• Evaporated Au
• No amorphous
material
Sputtering Damage?
Pt
Au
Resin
Au
Crystalline
feldspar
Pt

How are the feldspars
weathering?

Shap alkali feldspars: microtopography
(001)
Coherent
albite
lamellae
• Free of reaction products, but pitted
• Corrugations at sites of coherent albite lamellae

Shap alkali feldspars: microtopography
• Albite dissolves more rapidly
– Composition or magnitude of elastic coherency strain
– Higher strain on albite; dissolves faster

Shap alkali feldspars: etch pit networks
• Pits extend into grain interiors;
• “Etch tubes” - fluids exploit nanotunnels (Fitz Gerald et al. 2006)

Shap alkali feldspars: microstructure of pit walls
• Pits extend along former albite lamellae into the grain interior

Shap alkali feldspars: microstructure of pit walls
• Pit walls crystalline throughout; no amorphous layers

Glen Feshie
• Microbes, reaction products
• Pits dug in river terraces

Glen Feshie feldspars: weathering products
• Some amorphous weathering products; others 1.0 nm
Fe-aluminosilicate clays

Glen Feshie (80 yr): clay-feldspar interface
• Poorly crystalline material occurs between clays and
grain surface; little evidence for feldspar recrystallization:

Glen Feshie (1.1 kyr): reconstruction at grain surfaces
• Reaction products may have a gradational interface with
feldspar, possible interaction?

Glen Feshie (1.1 kyr): crystallization of clays
• Reaction products have crystallised K-aluminosilicates (illite)

Conclusions
• Shap weathers by stoichiometric
dissolution
– Centred on reactive sites
controlled by composition and/or
strain
– Leached layers must be < ~2 nm
• Glen Feshie weathering products
grow passively on grains from bulk
waters
– Some amorphous
– Crystallises to form clays

XPS on Shap feldspars
• 15°, 45°and 90°= 2.5, 6 and 9 nm
• Si enriched, K and Na depleted
~1-2 nm
• Surface chemistry modified
without destroying feldspar
framework (leaching of alkalis)
• Layer is present (1-2 nm) but
difficult to resolve on TEM.
XPS data represents “dilution
of result” with depth

TEM layers: a cautionary tale
• Previous TEM layers (tens of nm thick):
– Probable beam damage!
Amorphous
Au & Pt Au & Pt
50 nm 50 nm