  The z=0.9902 system along the quasar PG 1634+706 line of sight is a strong
MgII absorber (W(2796)>0.3A) with only weak CIV absorption (it is
``CIV-deficient''). To study this system, we used high-resolution spectra from
both HST/STIS (R=30,000) and Keck/HIRES (R=45,000). These spectra cover key
transitions, such as MgI, MgII, FeII, SiII, CII, SiIII, CIII, SiIV, and CIV.
Assuming a Haardt and Madau extragalactic background spectrum, we modeled the
system with a combination of photoionization and collisional ionization. Based
on a comparison of synthetic spectra to the data profiles, we infer the
existence of the following four phases of gas: i) Seven MgII clouds have sizes
of 1-1000pc and densities of 0.002-0.1/cm^3, with a gradual decrease in density
from blue to red. The MgII phase gives rise to most of the CIV absorption and
resembles the warm, ionized inter-cloud medium of the Milky Way; ii) Instead of
arising in the same phase as MgII, MgI is produced in separate, narrow
components with b~0.75km/s. These small MgI pockets (~100AU) could represent a
denser phase (~200/cm^3) of the interstellar medium (ISM), analogous to the
small-scale structure observed in the Milky Way ISM; iii) A ``broad phase''
with a Doppler parameter, b~60km/s, is required to consistently fit Ly-alpha,
Ly-beta, and the higher-order Lyman-series lines. A low metallicity (log Z <=
-2) for this phase could explain why the system is ``CIV-deficient'', and also
why NV and OVI are not detected. This phase may be a galactic halo or it could
represent a diffuse medium in an early-type galaxy; iv) The strong absorption
in SiIV relative to CIV could be produced in an extra, collisionally ionized
phase with a temperature of T~60,000K. The collisional phase could exist in
cooling layers that are shock-heated by supernovae-related processes.
