Presence of magnesium ion in the water column plays a key role in the precipitation of aragonite mineral, although it's not represented in the chemical composition of aragonite (CaCO3).
It has been usually related to the adsoprtion of Mg2+ ions onto calcite mineral surfaces, as this process inhibits calcite precipitation while it does not affect significantly to the nucleation and growth of aragonite crystals. This simple chart (and the references contained in it) may help you,