How do I make sure spice mixes don't make my curry grainy?

I'm making a curry, with an unknown dry mix of spices (smells remind me of cumin, turmeric and cinnamon) in the form of a powder.

When I've used it before though it's ended up grainy.

Have I been using to much spice mix? It have I not cooked the mix enough? How can I reduce the grainy texture?