  In a magnetized, collisionless plasma, the magnetic moment of the constituent
particles is an adiabatic invariant. An increase in the magnetic-field strength
in such a plasma thus leads to an increase in the thermal pressure
perpendicular to the field lines. Above a $\beta$-dependent threshold (where
$\beta$ is the ratio of thermal to magnetic pressure), this pressure anisotropy
drives the mirror instability, producing strong distortions in the field lines
on ion-Larmor scales. The impact of this instability on magnetic reconnection
is investigated using a simple analytical model for the formation of a current
sheet (CS) and the associated production of pressure anisotropy. The difficulty
in maintaining an isotropic, Maxwellian particle distribution during the
formation and subsequent thinning of a CS in a collisionless plasma, coupled
with the low threshold for the mirror instability in a high-$\beta$ plasma,
imply that the geometry of reconnecting magnetic fields can differ radically
from the standard Harris-sheet profile often used in simulations of
collisionless reconnection. As a result, depending on the rate of CS formation
and the initial CS thickness, tearing modes whose growth rates and wavenumbers
are boosted by this difference may disrupt the mirror-infested CS before
standard tearing modes can develop. A quantitative theory is developed to
illustrate this process, which may find application in the tearing-mediated
disruption of kinetic magnetorotational "channel" modes.
