  X-ray microtomography (XMT), X-ray diffraction (XRD) and magnetic hysteresis
measurements were used to determine micrometeorite internal structure,
mineralogy, crystallography, and physical properties at ~{\mu}m resolution. The
study samples include unmelted, partially melted (scoriaceous) and completely
melted (cosmic spherules) micrometeorites. This variety not only allows
comparison of the mineralogy and porosity of these three micrometeorite types,
but also reveals changes in meteoroid properties during atmospheric entry at
various velocities. At low entry velocities, meteoroids do not melt, and their
physical properties do not change. The porosity of unmelted micrometeorites
varies considerably (0-12%) with one friable example having porosity around
50%. At higher velocities, the range of meteoroid porosity narrows, but average
porosity increases (to 16-27%) due to volatile evaporation and partial melting
(scoriaceous phase). Metal distribution seems to be mostly unaffected at this
stage. At even higher entry velocities, complete melting follows the
scoriaceous phase. Complete melting is accompanied by metal oxidation and
redistribution, loss of porosity (1 $\pm$ 1%), and narrowing of the bulk (3.2
$\pm$ 0.5 g/cm$^{3}$) and grain (3.3 $\pm$ 0.5 g/cm$^{3}$) density range.
Melted cosmic spherules with a barred olivine structure show an oriented
crystallographic structure, whereas other subtypes do not.
