  The detailed polarization mechanisms of SiO masers originating from the near
circum\-stellar environment of Asymptotic Giant Branch stars are not yet
definitively known. Prevailing theories are broadly classified as either Zeeman
or non-Zeeman in origin, the latter including effects such as anisotropic
pumping or anisotropic resonant scattering. The predicted behavior of the
linear and circular polarization fractions and electric vector position angle
vary by theory. In particular, individual maser features that exhibit a
rotation in linear polarization of $\sim \pi/2$ as a function of frequency over
their extent can be utilized as a test of several maser polarization transport
theories. In this paper, we analyze one SiO ($\nu=1$, $J=1-0$) maser feature
toward the Mira variable, TX Cam that exhibits this internal polarization
rotation and persists across five epochs (spanning $\sim3$ months). We compare
our results to the predictions by several maser polarization theories and find
that the linear polarization across the feature is consistent with a geometric
effect for a saturated maser originating when the angle between the projected
magnetic field and the line of sight ($\theta$) crosses the Van Vleck angle
$\theta_F \sim 55^{\circ}$. However, the electric vector position angle (EVPA)
exhibits a smooth rotation across the spatial extent of the feature rather than
the expected abrupt $\pi/2$ flip. We discuss possible explanations for this
discrepancy and alternative theories. Circular polarization across the feature
is also analyzed and it is the most accurately described by Zeeman effects
giving rise to a circular polarization fraction of the form $m_c$ is
approximately proportional to $\cos \theta$.
