Which .xml file?
UPDATE: I found it in ImprovementDefs.xml
(C:\Programfiles (x86)\Steam\steamapps\common\Galactic Civilizations III\data\game\ImprovementDefs.xml)
in the <Improvement> stanza.
I removed all blanks after XenoIrrigation (I believe about seven instances). But when I ran the game the zeno irragation tiles were still invisible. Is there another file that also needs to be corrected or does this file have to be re-compiled into the program?