Alter index ??????

I am getting an error on doing a large sql_load into an empty table that has been indexed:

	ora-01654: unable to extend index sys.grades_index by 3596 in
tablspace 	user_data.

	I am trying to enlarge the index but get an error that I am using an
invalid 		option;

	alter index student.grades_index
	allocate extent size 500K
	datafile 'i:\storage.ndx';

	the datafile is created and I have confirmed it. I am using version 7.1
is this 	the problem??????????
							Thanks,
								Bob