The interpretation of silicon isotope data for quartz is hampered by the lack of experimentally determined fractionation factors between quartz and fluid. Further, there is a large spread in published oxygen isotope fractionation factors at low temperatures, primarily due to extrapolation from experimental calibrations at high temperature. We present the first measurements of silicon isotope ratios from experimentally precipitated quartz and estimate the equilibrium fractionation vs. dissolved silica using a novel in situ analysis technique applying secondary ion mass spectrometry to directly analyze experimental products. These experiments also yield a new value for oxygen isotope fractionation. Quartz overgrowths up to 235 μm thick were precipitated in silica–H₂O–NaOH–NaCl fluids, at pH 12–13 and 250 °C. At this temperature, 1000lnα³⁰Si(Qtz–fluid) = 0.55 ± 0.10‰ and 1000lnα¹⁸O(Qtz–fluid) = 10.62 ± 0.13‰, yielding the relations 1000lnα³⁰Si(Qtz–fluid) = (0.15 ± 0.03) * 10⁶/T² and 1000lnα¹⁸O(Qtz–fluid) = (2.91 ± 0.04) * 10⁶/T² when extended to zero fractionation at infinite temperature. Values of δ³⁰Si(Qtz) from diagenetic cement in sandstones from the basal Cambrian Mt. Simon Formation in central North America range from 0 to − 5.4‰. Paired δ¹⁸O and δ³⁰Si values from individual overgrowths preserve a record of Precambrian weathering and fluid transport. The application of the experimental quartz growth results to observations from natural sandstone samples suggests that precipitation of quartz at low temperatures in nature is dominated by kinetic, rather than equilibrium, processes.