Here's an interesting article about cacti potting mix with focus on mineral soil and difficult Mexican cacti. Of course it is written for the European cactus collector, however, I find it very interesting no matter where you grow your plants. My only contribution was the English translation.
The author is a dear friend of mine Dag Panco de Grid (aka Lupul, Le Loup, and similar nicknames meaning wolf), a real connoisseur of (especially) Mexican cacti who had a fabulous collection (now transferred to a French collector http://www.lafermeauxcactus.com/ - almost half of the plants in this collection were purchased from Dag). Although he pursued other interest in life (writing fiction) and gave up collecting cacti few years ago, he is now the curator of the most interesting private Romanian C&S collections - Basarab Popa of Constanta (a city at the Black Sea), restructuring and reshaping the entire collection, etc., he has very ambitious plans and now also the financial support he lacked in the past.
I hope you'll find it interesting, please also feel free to distribute the file. Enjoy!