Minerals in the sepiolite-palygorskite series, (Mg8-y-zR3+y?z)[Si12-x R3+xO30](OH)4(OH2)4·R2+(x-y+2z)2(H2O)8-(Mg5-y-zR3+y?z)[Si8-xR3+xO20] (OH)2(OH2)4·R2+(x-y+2x)/2(H2O)4, are commonly precipitating directly from oversaturated surface-near water masses and thus they have been extensively used for the reconstruction of sedimentary facies.In order to gain a better understanding of the processes leading to sepiolite formation in natural surroundings, sepiolite was experimentally precipitated at 25 5 C by direct precipitation from Si-doped seawater and Si-doped synthetic MgCl2 brines (both undersaturated in respect to amorphous opal-A and/or brucite/Mg(OH)2) at molar Si:Mg ratios of 1:27.5, 1:55, and 1:110. Over a period of 37 up to 91 days, changes in solution chemistry and solid-phase composition were monitored by using hydrogeochemical modeling, UV-vis spectroscopy and ICP-OES analysis as well as XRD, FT-IR, and TEM techniques.The experimental precipitates were identified as Mg-rich phyllosilicate with a modulated structure, which is mineralogically, structurally and geochemically identical to natural occurring sepiolite. Sepiolite is the only precipitate thus the apparent precipitation rates of sepiolite from seawater and synthetic MgCl2 brine (Si:Mg=1:27.5) were calculated to be 7.3·10-14 mol/s and 2.8·10-13 mol/s, respectively. At the higher Si:Mg ratios of 1:55 and 1:110 the apparent precipitation rate of sepiolite increased to 5.6·10-13 mol/s and 1.2·10-12 mol/s, respectively.The estimated crystal growth rates suggest that sepiolite precipitation from Si-doped seawater is kinetically inhibited, most probably due to the formation of uncharged MgSO40 aquo-complexes through reaction of free Mg2+ ions with seawater sulfate, as indicated by hydrogeochemical model-ling. This might explain why sepiolite-palygorskite group minerals are relatively rare in many modern and ancient marine sediments.