This fine surface is then carbon coated using the Vacuum Evaporator to
facilitate electron transport and prevent accumulation on the surface of
the sample. The samples were placed under a vacuum bell and the chamber
pumped down to 1x10-4 mbar. Electrical current was used to
excite a carbon electrode, causing it to eject carbon atoms which
condensed on the samples.
The thin section was placed in a brass sample holder and carbon tape was
applied to stablize the sample and insure electrical connection between
the carbon-coated surface and the brass sample holder. The sample was
inserted into the specimen exchange chamber, and after evacuation, mounted
on the platform inside the column.
Microanalysis was performed using "Link ISIS" software. Image Setup was
set to Slow Acquisition Speed and Fine Resolution to collect
the images. To prepare the electron beam to data acquisition parameters,
the current was adjusted to -.6000+.0025nA by adjusting the
Condenser Lens settings. Point analyses were performed on multiple
crystals and parts of crystals visible in each image analyzed. Spectra
were collected, maintaining dead time during scan of 30-40%.
Quantitative analyses were calculated using the SEMQuant function.
Combined element by Stoichiometry was used by specifying the
Number of ions based on qualitative visual analysis of the spectra.
In many cases, several analyses were calculated to determine the best fit
of the data to known mineral formulas and/or approximate formulas.
... sample preparation ...
Thin sections are prepared of the rock samples by fixing a billet of the
rock to a glass slide and then lapping it to 30 microns in thickness. In
order to prepare the thin section for SEM analysis, it is polished down to
a 1 micron grit.
... analysis ...
The SEM was booted at 20kV acceleration voltage. Start up parameters
include:
The electron beam was saturated per the course manual1 and
focused using the sample stage z-adjustment (vertical adjustment) knob in
order to maintain the lens parameters listed above. The SEM was switched
into "composition" or backscattered image mode to examine the sample.