Silicate mineralogy of the dust in the inner coma of comet C/1995 01 (Hale-Bopp) pre- and post-perihelion

We present 7.5-13.4 mu m infrared (IR) spectrophotometry (R similar or equa l to 180-360) of the 10 mu m silicate emission from dust in the inner coma (i.e., within a diameter of 3") of comet C/1995 O1 (Hale-Bopp) at four temp oral epochs from 1996 October through 1997 June during Hale-Bopp's approach to, arrival at, and recession from perihelion. Hale-Bopp's silicate featur e is the strongest observed from any comet to date: the flux-to-continuum r atio at 10.0 mu m is 2.5 at 2.8 AU preperihelion, increases to 3.0 at 0.93 AU, and then decreases to 2.4 at 1.7 AU postperihelion, dropping more rapid ly in strength than expected from preperihelion spectra and indicating a di minishment in the relative abundance of submicron sized grains by perihelio n passage. The silicate feature also evolves with heliocentric distance. Wh en far from perihelion, at similar to 2.8 AU, the High Efficiency Faint Obj ect Grating Spectrometer (HIFOGS) 10 mu m silicate feature contains a 9.3 m u m shoulder attributable to amorphous pyroxene, broad emission from amorph ous olivine (9.7 mu m), and the 11.2 mu m peak associated with crystalline olivine. Concurrent with the HIFOGS spectra at 2.8 AU, the Infrared Space O bservatory (ISO) Short Wavelength Spectrometer (SWS) spectrum also shows th e strong far-IR peaks of crystalline olivine at 18 mu m, 23 mu m, and 33 mu m. When close to perihelion the HIFOGS spectra include a newly discovered 9.3 mu m peak, identifiable as Mg-rich crystalline pyroxene. We hypothesize that the crystalline pyroxenes are cooler than the other silicates, too co ol to be detected at 9.3 mu m at 2.8 AU but warm enough to be detected at 1 .2 AU. The best-fit empirical model for the temporal evolution of Hale-Bopp 's silicate feature constrains the Mg-rich crystalline pyroxenes to be domi nated by ortho-pyroxene rather than clino-pyroxene. The crystalline pyroxen es have a color temperature that is 0.6 times cooler than and an abundance that is about 9 times greater than the other silicates at all epochs, assum ing equivalent particle size distributions for all components. Comparing cr ystalline ortho-pyroxene and crystalline olivine in radiative equilibrium, both with Mg number 09,the ortho-pyroxene crystals are cooler because they are less absorbing at visible acid near-IR wavelengths. Thus, it is the hig h Mg content of the crystalline pyroxenes that accounts for their cooler te mperature. The dominance of Mg-rich pyroxenes among Hale-Bopp's cometary si licates is consistent with PUMA-I mass spectrometer measurements of comet P /Halley 1986 III and with cometary interplanetary dust particles. The high Mg content of the pyroxenes in comet Hale-Bopp implies that they are either pristine solar nebula condensates or presolar grains such as the Mg-rich c rystals recently discovered by ISO around asymptotic giant branch stars.