Infrared Spectra of Pyroxenes (Crystalline Chain Silicates) at Room
  Temperature
  Pyroxene crystals are common in meteorites but few compositions have been
recognized in astronomical environments. We present quantitative
room-temperature spectra of 17 Mg-- Fe-- and Ca--bearing ortho- and
clinopyroxenes, and a Ca-pyroxenoid in order to discern trends indicative of
crystal structure and a wide range of composition. Data are produced using a
Diamond Anvil Cell: our band strengths are up to 6 times higher than those
measured in KBr or polyethylene dispersions, which include variations in path
length (from grain size) and surface reflections that are not addressed in data
processing. Pyroxenes have varied spectra: only two bands, at 10.22~$\mu$m and
15.34~$\mu$m in enstatite (En$_{99}$), are common to all. Peak-wavelengths
generally increase as Mg is replaced by Ca or Fe. However, two bands in
MgFe-pyroxenes shift to shorter wavelengths as the Fe component increases from
0 to 60 per cent. A high-intensity band shifts from 11.6~$\mu$m to 11.2~$\mu$m
and remains at 11.2~$\mu$m as Fe increases to 100~per~cent; it resembles an
astronomical feature normally identified with olivine or forsterite. The
distinctive pyroxene bands between 13~ and 16~$\mu$m show promise for their
identification in MIRI spectra obtained with JWST. The many pyroxene bands
between 40 and 80~$\mu$m could be diagnositic of silicate mineralogy if data
were obtained with the proposed SPICA telescope. Our data indicate that
comparison between room-temperature laboratory bands for enstatite and cold
$\sim 10-K$ astronomical dust features at wavelengths $\gtrsim 28~\mu$m can
result in the identification of (Mg,Fe)- pyroxenes that contain 7--15 % less
Fe-- than their true values because some temperature shifts mimic some
compositional shifts. Therefore some astronomical silicates may contain more
Fe, and less Mg, than previously thought.
