  Exoplanet atmosphere characterisation has become an important tool in
understanding exoplanet formation, evolution. However, clouds remain a key
challenge for characterisation: upcoming space telescopes (e.g. JWST, ARIEL)
and ground-based high-resolution spectrographs (e.g. CRIRES+) will produce data
requiring detailed understanding of cloud formation and cloud effects. We aim
to understand how the micro-porosity of cloud particles affects the cloud
structure, particle size, and material composition. We examine the
spectroscopic effects of micro-porous particles, the particle size
distribution, and non-spherical cloud particles. We expanded our kinetic
non-equilibrium cloud formation model and use a grid of prescribed 1D
(T_gas-p_gas) DRIFT-PHOENIX profiles. We applied the effective medium theory
and the Mie theory to model the spectroscopic properties of cloud particles
with micro-porosity and a derived particle size distribution. We used a
statistical distribution of hollow spheres to represent the effects of
non-spherical cloud particles. Highly micro-porous cloud particles (90% vacuum)
have a larger surface area, enabling efficient bulk growth higher in the
atmosphere than for compact particles. Increases in single-scattering albedo
and cross-sectional area for these mineral snowflakes cause the cloud deck to
become optically thin only at a wavelength of ~100 ${\rm \mu m}$ instead of at
the ~20 ${\rm \mu m}$ for compact cloud particles. A significant enhancement in
albedo is also seen when cloud particles occur with a locally changing Gaussian
size distribution. Non-spherical particles increase the opacity of silicate
spectral features, which further increases the wavelength at which the clouds
become optically thin. JWST MIRI will be sensitive to signatures of
micro-porous and non-spherical cloud particles based on the wavelength at which
clouds are optically thin.
