LDA VolumeViz is an original file format created by Mercury Systems .
There is no open document with the schema file but you can get information from the Open Inventor site about VolumeViz and a bit explanation about the format from the Open Inventor Developer Zone, What is VolumeViz and LDM Data
All capters are part of the book Open Inventor Mentor, 2nd Edition - Volume II
I hope this helps you.