Magnesite is essential to fertilizer, paper, glass, construction; most recently as eco-cement, at Mt-Gt scales. Mg2+ sources are plentiful recoverable via mineral carbonation of Mg-silicate, whose deposits are @ 10^5 Gt with magnesite being the most stable Mg-carbonate under all conditions. Yet, MgCO3-mineralisation is retarded by Mg-dehydration. Industrially magnesite is produced with high-T/P condition, unlike ambient formation in nature, accelerated by impurities in ground water (HS-, SiO4-2, RCOO-), as-yet irreproducible in the lab. Geochemical evidence indicates these electrolyte solutions catalysing magnesite formation in its 3 principal steps: 1.Mg-dehydration, 2. Nucleation, 3. growth. We propose to undertake the 1st ever neutron measurements of in-situ Mg-carbonation in industrially relevant slurries. Herein, NCS will probe changing bonding regime throughout magnesite formation