I've used the mineral blocks before but they don't seem to eat it as well as the white blocks but maybe I should change back. I've also heard the argument that you should use loose salt as opposed to a block because they can make their tongue sore from licking before they get the level of salt they need. Mine usually end up biting the block and that wears their teeth down. I've tried the loose mineral salt before, but it sure is messy. They usually end up turning over the pan that it's in and wasting most of it. Maybe if I put it in a bucket mounted on the wall it would work better.
I think with the mineral blocks, they really don't need a lot. They really ~shouldn't~ have so much at one time. I've seen horses who have been a long time without one lick it compulsively for a day or two, and then not so often, as if they actually have an inner gauge that tells them they are low, and when they don't need it any more.
Perhaps in your hot hot climate where they sweat more, provide both plain salt and a mineral block.
I've used that mineral powder before. My vet said to just top dress their feed with it, about 2 ounces a day.
Becky