Olivine is a volcanic rock. It is classified along with basalt as mafic, meaning that the dust will be high in magnesium and iron. Unlike basalt, which typically has around 2-3% magnesium, olivine will have composition percentages upwards of 50%.
This dust should be used sparingly and for the express purpose of adding magnesium to a potting mix or garden soil. The magnesium, along with the other trace elements will release slowly and does not easily leach away.