Dolomite (CaMg(CO3)2) forms in minor quantities in modern environments yet comprises most of the Precambrian carbonate rock record. Precambrian dolomites are often fine-grained and fabric-retentive and are interpreted to have precipitated as primary cements or formed as early diagenetic replacements of CaCO3. Detailed physical and chemical characterization of these dolomites could inform their origin and relevance for paleoenvironmental reconstruction. Here, we use synchrotron radiation to produce a nanometer-resolution crystal orientation map of one exquisitely-preserved ooid deposited at the onset of the Shuram carbon isotope excursion (~574 Ma). The crystal orientation map reveals small (~10μm) acicular, radially-oriented crystals grouped into bundles of similarly-oriented crystals with varying optical properties. We interpret that this dolomite formed via primary, spherulitic precipitation during ooid growth in shallow marine waters. 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.