what data type to store a json array in mysql (5.6) database

You can use text datatype to store JSON data in MySQL Database.

It depends on the length of the JSON data your are going to store. If it's not too long you can use VARCHAR, but with this you have the limit of 64K:

Manual says: The length can be specified as a value from 0 to 65,535. The effective maximum length of a VARCHAR is subject to the maximum row size (65,535 bytes, which is shared among all columns) and the character set used.

So if you expect to have huge objects, use any of the TEXT types:

TEXT: 65,535 characters - 64 KB 
MEDIUMTEXT: 16,777,215 - 16 MB 
LONGTEXT: 4,294,967,295 characters - 4 GB 

Since Mysql 5.7.8 you're able to use the native JSON data type, though.