It's used in soil mixes for trace elements - some very interesting research out there about it. This guy did a germ test and found up to 80% more root mass using it for some conditions (compost+rockdust), plus his germ room sounds a lot like ours
http://www.organicgarden.org.uk/remi...tion_trial.htm
I just wondered if a sprinkle in coco would bring any benefit, or just get washed through, or is pointless with coco nutes... I'm going to repot my mud dwellers into a mix with some rockdust in though.