Sorry for your loss. Raising chickens has quite a steep learning curve and I think most of us don't know much about chickens or all the things that can go wrong when we get them (I know I didn't have a clue...). And of course, even when you do have a good understanding you're still going to lose some birds despite your best efforts.
As for grit, they are probably finding what they need on the ground if you've got some rocky dirt for them to scratch through, though they do need it everyday, so if sometimes they are stuck in a coop that doesn't have any sort of grit available that could be a problem. The crushed granite that you can buy for grit here is around 1/4 of an inch for grown birds.