  We present optical spectroscopic and photometric observations of Type Ia
supernova (SN) 2006X from --10 to +91 days after the $B$-band maximum. This SN
exhibits one of the highest expansion velocity ever published for SNe Ia. At
premaximum phases, the spectra show strong and broad features of
intermediate-mass elements such as Si, S, Ca, and Mg, while the O{\sc
i}$\lambda$7773 line is weak. The extremely high velocities of Si{\sc ii} and
S{\sc ii} lines and the weak O{\sc i} line suggest that an intense
nucleosynthesis might take place in the outer layers, favoring a delayed
detonation model. Interestingly, Si{\sc ii}$\lambda$5972 feature is quite
shallow, resulting in an unusually low depth ratio of Si{\sc ii}$\lambda$5972
to $\lambda$6355, $\cal R$(Si{\sc ii}). The low $\cal R$(Si{\sc ii}) is usually
interpreted as a high photospheric temperature. However, the weak Si{\sc
iii}$\lambda$4560 line suggests a low temperature, in contradiction to the low
$\cal R$(Si{\sc ii}). This could imply that the Si{\sc ii}$\lambda$5972 line
might be contaminated by underlying emission. We propose that $\cal R$(Si{\sc
ii}) may not be a good temperature indicator for rapidly expanding SNe Ia at
premaximum phases.
