  Airless planetary bodies are directly exposed to space weathering. The main
spectral effects of space weathering are darkening, reduction in intensity of
silicate mineral absorption bands, and an increase in the spectral slope
towards longer wavelengths (reddening). Production of nanophase metallic iron
(npFe$^{0}$) during space weathering plays major role in these spectral
changes. A laboratory procedure for the controlled production of npFe$^{0}$ in
silicate mineral powders has been developed. The method is based on a two-step
thermal treatment of low-iron olivine, first in ambient air and then in
hydrogen atmosphere. Through this process, a series of olivine powder samples
was prepared with varying amounts of npFe$^{0}$ in the 7-20 nm size range. A
logarithmic trend is observed between amount of npFe$^{0}$ and darkening,
reduction of 1 {\mu}m olivine absorption band, reddening, and 1 {\mu}m band
width. Olivine with a population of physically larger npFe$^{0}$ particles
follows spectral trends similar to other samples, except for the reddening
trend. This is interpreted as the larger, ~40-50 nm sized, npFe$^{0}$ particles
do not contribute to the spectral slope change as efficiently as the smaller
npFe$^{0}$ fraction. A linear trend is observed between the amount of
npFe$^{0}$ and 1 {\mu}m band center position, most likely caused by Fe$^{2+}$
disassociation from olivine structure into npFe$^{0}$ particles.
