The mineral dolomite (CaMg(CO3)2) forms in minor quantities in few modern sedimentary environments yet comprises the bulk of the Precambrian carbonate rock record. Precambrian dolomites are often preserved as fine-grained mudstone or in fabric-retentive forms, which possibly precipitated from minimally-altered seawater as primary cements or formed as replacements of primary CaCO3 in early diagenetic environments. Direct evidence for primary dolomite precipitation from seawater in depositional environments has not yet been described. Here, we use synchrotron radiation to produce a nanoscale-resolution crystal orientation map of one ooid grain deposited at the onset of the Shuram carbon isotope excursion at ca. 574 Ma. In thin section, this ooid appears to preserve exquisite detail. Crystal orientation maps reveal small (~10 µm) acicular, radially-oriented crystals grouped into bundles of similarly-oriented crystals that are variably length-fast and length-slow. We interpret that this petrographic fabric formed as the result of primary, spherulitic dolomite precipitation during ooid growth in shallow marine waters at ca. 574 Ma. This result provides additional evidence that the physicochemical properties of late Precambrian oceans promoted dolomite precipitation and supports a primary origin for the Shuram excursion.