I have been having trouble polishing a black marble floor. I cannot seem to prevent the polishing powder from etching areas that I just completed. How can I prevent the etching?

This is a common problem with many of the marble polishing powders because they all contain oxalic acid. Acids of course will etch marble. To prevent etching you need to wet the areas you are not working on with water and the addition of either a stone soap or ammonia. If using ammonia, start with about one cup per gallon of water. Be sure to put enough water on the dry marble. The ammonia will neutralize the acid and etching will be prevented.
You will also need to make sure that your polishing slurry does not dry. Keep the slurry as wet as possible. Be sure you are using a splash guard on your polishing machine. Splash guards can be made with plastic landscape edging or flexible plastic baseboard. These materials are available at your local home center or hardware store.