a raw material with a high content of aluminum oxide has the following composition , in weight - percent , based on dry weight : al 2 o 3 50 – 80 % mgo 2 – 15 % sio2 1 – 15 % cao 0 . 5 – 20 % fe 2 o 3 0 . 5 – 2 % na 2 o 0 . 5 – 2 % al 0 . 1 – 2 % aln 0 . 1 – 1 % k 2 o 0 . 1 – 1 . 5 % f 0 . 1 – 2 % cl 0 . 1 – 0 . 8 % balance in total , not to exceed 5 % loss on ignition , not to exceed 15 % the main mineral constituents of the raw material are , in weight - percent aluminum hydroxide 20 – 60 % corundum α - al 2 o 3 10 – 40 % spinel mgal 2 o 4 5 – 40 % specifically , the aluminum hydroxide is present as aluminum mono - hydroxide al 2 o 3 . h 2 o and aluminum tri - hydroxide al 2 o 3 . 3h 2 o in a weight ratio exceeding 0 . 25 .