How can I identify "tough" (mixed) granites?
So, the question is how can I tell if a minedat description of a plutonic rock is favorable according to the criteria of the quoted passage?


No. The description doesn't say how much feldspar is in the rock. It could be 5%, or it could be 20%. It also doesn't say anything about the iron content of the feldspar (although with some petrological hand waving you can assume by the presence of other minerals).

The nepheline syenite is going to have no feldspar (or very little), because it's usually either nepheline or feldspar, not both. But, we don't know whether nepheline is any good for the purpose.

I'm also wondering why is hornblende considered superior because it is a mineral that cleaves and breaks very easily.

One thing that is important is that they stress the fine grain size. Usually rocks with fine grain size are tougher than those with coarser.