The main ore from which alumina oxide products are refined, contains diaspore, gibbsite and boehmite minerals. LOI is 25%.
The Bayer refining process of turning Bauxite into alumina used by alumina refineries worldwide involves four steps - grinding and digestion, clarification, precipitation and calcination. Alcoa describes the process like this:
"To turn bauxite into alumina, we grind the ore and mix it with lime and caustic soda, pump this mix into high-pressure containers, and heat it. The aluminum oxide is dissolved by the caustic soda, then precipitated out of this solution, washed, and heated to drive off water. What is left is the white powder called alumina, which is transformed into aluminum metal in the smelting process."
Bauxite at Wikipedia
Micrograph of gibbsite at minersoc.org