  We report on ASCA energy spectra of Cir X-1 taken near its zero phase on 1994
August 4-5. The ASCA SIS detectors allow a more detailed study of the iron K
complex than has been possible before. We find that prior to a sudden upward
flux transition the dominant iron K feature appears to consist of a large edge
from neutral or nearly-neutral iron. The depth of the edge corresponds to an
absorption column of about 1.5E24 cm^{-2}, while little absorption over that
expected from the Galaxy is seen at lower X-ray energies. The differential
absorption at high and low X-ray energies combined with the iron edge energy
are strong evidence that partial covering is a crucial determinant of the
behaviour observed from Cir X-1. The continuum spectral variability observed by
ASCA can also be understood naturally in terms of partial covering column
changes. There is evidence for a relatively weak emission line from neutral
iron with an equivalent width of only about 65 eV. After the flux transition,
the strength of the edge feature is greatly reduced, suggesting a large
reduction in the amount of partial covering. For a large region of
statistically acceptable chi-squared parameter space, the luminosity of Cir
X-1, after correction for partial covering, need not change during the
transition. We discuss models for the partial covering and suggest that X-ray
scattering by electrons may be important. Aspects of the Cir X-1 spectrum are
very similar to those of Seyfert 2 galaxies with Compton-thin tori.
