How to get rid of a corrupt block that has CORRUPTION_TYPE of FRACTURED on V$DATABASE_BLOCK_CORRUPTI

We've got a corrupt block as follows on a DEV database that doesn't have RMAN backups.

SQL> select * from v$database_block_corruption;

     FILE#     BLOCK#     BLOCKS CORRUPTION_CHANGE# CORRUPTIO
---------- ---------- ---------- ------------------ ---------
         6     781711          1                  0 FRACTURED

SQL> select segment_type, owner||'.'||segment_name
  2  from dba_extents
  3  where file_id = 6 and 781711 between block_id
  4  and block_id+blocks -1;

SEGMENT_TYPE       OWNER||'.'||SEGMENT_NAME
------------------ ----------------------------------------------------------------------------------------------------------------
INDEX              SOAINFRA.MEDIATOR_INSTANCE_INDEX1

I've tried rebuilding the index to see if that will get rid of the corrupt block, but it's still saying corrupt, so I dropped it and then looked at the corrupt block again.