  Direct mid-infrared signatures of silicate clouds in substellar atmospheres
were first detected in Spitzer observations of brown dwarfs, although their
existence was previously inferred from near-infrared spectra. With the
Mid-Infrared Instrument (MIRI) instrument on JWST, we can now more deeply probe
silicate features from 8 to 10 microns, exploring specific particle
composition, size, and structure. Recent characterization efforts have led to
the identification in particular of silica (silicon dioxide, SiO$_2$) cloud
features in brown dwarfs and giant exoplanets. Previous modeling, motivated by
chemical equilibrium considerations, has primarily focused on magnesium
silicates (forsterite, enstatite), crystalline quartz, and amorphous silica to
match observations. Here, we explore the previously neglected possibility that
other crystalline structures of silica, i.e. polymorphs, may be more likely to
form at the pressure and temperature conditions of substellar upper
atmospheres. We show how these polymorphs may be distinguished from each other
with current JWST observations. We also explore how such particles could form
and be dynamically lofted and sedimented throughout the atmosphere, and
implications for the underlying chemical and dynamical processes governing
these objects. We ultimately propose that accounting for the distinct opacities
arising from the possible crystalline structure of cloud materials may act as a
powerful, observable diagnostic tracer of atmospheric conditions, where
particle crystallinity records the history of the atmospheric regions through
which clouds formed and evolved. Finally, we highlight that high fidelity,
accurate laboratory measurements of silica polymorphs are critically needed to
draw meaningful conclusions about the identities and structures of clouds in
substellar atmospheres.
