Table schema:
CREATE TABLE table_name_42 (sio_2 INTEGER, na_2_o VARCHAR, al_2_o_3 VARCHAR)

Question: What is average SiO 2, when Na 2 O is 7.53, and when Al 2 O 3 is less than 2.13?
SQL: SELECT AVG(sio_2) FROM table_name_42 WHERE na_2_o = 7.53 AND al_2_o_3 < 2.13