  We implement a technique to characterize electromagnetic properties at
frequencies 100 to 165 GHz (3 cm$^{-1}$ to 4.95 cm$^{-1}$) of oriented
montmorillionite samples using an open cavity resonator connected to a
sub-millimeter wave VNA (Vector Network Analyzer). We measured dielectric
constants perpendicular to the bedding plane on oriented Na$^{+}$ and
Ca$^{++}$-ion stabilized montmorillionite samples deposited on a glass slide at
ambient laboratory conditions (room temperature and room light). The clay layer
is much thinner ($\sim$ 30 $\mu$m) than the glass substrate ($\sim$ 2.18 mm).
The real part of dielectric constant,$\epsilon_{re}$, is essentially constant
over this frequency range but is larger in Na$^{+}$- than in Ca$^{++}$-ioned
clay. The total electrical conductivity (associated with the imaginary part of
dielectric constant, $\epsilon_{im}$) of both samples increases monotonically
at lower frequencies ($<$ 110 GHz), but shows rapid increase for Na$^{+}$ ions
in the regime $>$ 110 GHz. The dispersion of the samples display a dependence
on the ionic strength in the clay interlayers, i.e., $\zeta$-potential in the
Stern layers.
