Transmission electron microprobe analysis
Whole grains of sand were embedded in epoxy and subsequently
polished to half their thickness. These mounts were carbon coated
and then examined in a JEOL 8600 electron microprobe. Areas were
chosen for further examination in the transmission electron microscope
(TEM). The mounts were glued to a glass slide and mechanically
ground to a thickness of ~30 μm. Copper TEM grids were glued to
sand grain sections with Crystalbond adhesive, and removed from
the glass slide. The TEM grid was thinned in a Gatan Duo Ar Ion Mill
to perforation. The TEM grids were then carbon coated and examined
in a Philips CM 300 FEG (field emission gun) at 297 kV and a Philips
EM 420 at 120 kV. Conventional TEM and selected area electron diffraction
(SAED) methods were used to determine the crystalline nature
of the phases at the quartz/coating boundaries (EM 420).
Scanning (S)TEM methods were used to obtain X-ray maps for elemental
distributions (CM 300 FEG). X-rays were collected using a
light element Oxford energy dispersive spectrometer (EDS) and
processed by the software ESVision 4. Dwell time for X-ray acquisition
was 1 s/pixel. At each pixel an entire X-ray spectrum was collected
which is then available for data mining. Simultaneously, an
annular dark-field (ADF) image was collected which is predominantly
sensitive to mean atomic number and mass thickness.