Seawater, geothermal brines, and industrial wastes are abundant and mineral rich. However, selectively extracting minerals and elements of interest and commercial value from such non-traditional sources can be a challenge. Current approaches to mineral extraction are chemical- and energy-intensive. As an alternative, we present a simple, single-step, separation technique that leverages non-equilibrium conditions in a flow cell to generate phase-pure solid precipitates. The technique relies on differences in solubility constants to selectively separate minerals from aqueous ion mixture (e.g., seawater). The method can be used to extract various minerals deemed critical by the DOE, by simply selecting the appropriate reactant to flow along with source water. Here we demonstrate the approach for selective separation of Mg(OH)2 from seawater (Instant Ocean) using NaOH as the reactant solution. We experimentally show that using the flow-cell approach generates phase pure Mg(OH)2 compared with bulk mixing of the same two solutions (reactant NaOH and seawater). Our data is obtained using a microfluidics device, however, the same concept of laminar flow for achieving non-equilibrium products has been previously utilized at industrial scale confirming practical relevance of the technique. Further, the method can be used to extract multiple minerals from source water by appropriately sequencing reactants.