This is one simple petrogenetic grid.
The letters in this diagram are shortcuts for complex chemical formulas.
If I am not mistaken grt stand for R3 R2 [SiO4]3 for example.
Where first R can stand for Mg, Fe, Mn or Ca
Second R can be Al, Fe, Cr, Ti or Zr
While Si and O form the basic of mineral structure.
The shortcuts are simbols for chemical elements.