In the Ordos area of North China, the Ordovician consists of dolostones and limestones deposited on a broad shallow carbonate platform. In the southeast part to the study area, a special type of dolostone characterized by wheat dolomite crystals (WDC for short) is developed in the Middle Ordovician Majiagou 4th Formation. Restricted within the southeast part of the study area and covering an area of about 20000 km2, these dolostones are closely associated with gypsum/anhydrite rocks. The wheat dolomite crystals are elongated rhombohedra different from normal rhombohedral dolomite crystals. The optic axis c of the wheat dolomite crystals is at right angles to their elongation. In this respect, such crystals are similar to anhydrite crystals. The degree of order of wheat dolomite crystals is from 0.52 to 0.60, averaging 0.56; the molar concentration of CaCO3is from 51.22 to 52.05%, averaging 51. 64%; theδ13C is from -0.12‰to -0.20‰(PDB), averaging 0.16‰; theδ18O is from -5.75‰to -6.28‰(PDB), averaging -6.02‰; Fe2+is from 2980 to 3140×10-6,averaging 3063×10-6; Mn2+is from 83 to 105×10-6, averaging 94×10-6; K+is from 1200 to 1410×10-6, averaging 1305×10-6; and Na+is from 840 to 850×10-6, averaging 845×10-6. In the study area, the dolostones consisting mainly of wheat dolomite crystals were formed by SO2-4_rich brine through deep burial dolomitization of open marine limestones of the Ordovician Majiagou 4th Formation. It was the presence of SO2-4that made dolomite crystals grow like anhydrite crystals. The sulphate ions were derived from dissolution of gypsum/anhydrite deposits in adjacent strata.