Fused silica (FS) is transparent to some wavelengths in the infrared, but it's limited. For a wider transparency window, especially at longer wavelengths, optical-grad silicon or germanium is used (e.g. FLIR cameras). Also, there are likely to be antireflection coatings which further limits the use of the FS elements away from their optimized wavelength range.
This was probably done "just in case" someone would like to try IR, and it's good thinking, but I'm wondering if these have every been used in orbit, even for testing purposes.
This question has a new companion: Why do the LAGEOS' satellites have four germanium corner cube reflectors out of over 400?
below: Sample output from JPL Horizons.
Revised: Sep 11, 2017 LAGEOS-2 -122195 LAGEOS-2 (1992-070B, "Laser Geodynamics Satellite", NASA/ASI) 60-cm diameter sphere, 426 corner cube reflectors (VIS), 4 germanium (IR) Deployed: 1993-Oct-22 from Space Shuttle Columbia (STS 52)
below: From here.