We report the first detections of the repeating fast radio burst source FRB 121102 above 5.2 GHz .
Observations were performed using the 4 - 8 GHz receiver of the Robert C. Byrd Green Bank Telescope with the Breakthrough Listen digital backend .
We present the spectral , temporal and polarization properties of 21 bursts detected within the first 60 minutes of a total 6-hour observations .
These observations comprise the highest burst density yet reported in the literature , with 18 bursts being detected in the first 30 minutes .
A few bursts clearly show temporal sub-structures with distinct spectral properties .
These sub-structures superimpose to provide enhanced peak signal-to-noise ratio at higher trial dispersion measures .
Broad features occur in \sim 1 GHz wide subbands that typically differ in peak frequency between bursts within the band .
Finer-scale structures ( \sim 10 - 50 MHz ) within these bursts are consistent with that expected from Galactic diffractive interstellar scintillation .
The bursts exhibit nearly 100 % linear polarization , and a large average rotation measure of 9.359 \pm 0.012 \times 10 ^ { 4 } rad m ^ { -2 } ( in the observer ’ s frame ) .
No circular polarization was found for any burst .
We measure an approximately constant polarization position angle in the 13 brightest bursts .
The peak flux densities of the reported bursts have average values ( 0.2 \pm 0.1 Jy ) , similar to those seen at lower frequencies ( < 3 GHz ) , while the average burst widths ( 0.64 \pm 0.46 ms ) are relatively narrower .