Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: 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?

### Accurate SQL ###

SELECT AVG(sio_2) FROM table_name_42 WHERE na_2_o = 7.53 AND al_2_o_3 < 2.13