Abstract Illite from Muloorina, just south of Lake Eyre in South Australia, is commonly used as a standard ferric iron-bearing illite which has a cation exchange capacity (CEC) of ~22 meq/100 g, too high a value for this clay to conform to the most recent conclusions about illite composition. The objective of the present study was to reassess the Muloorina illite to determine the reason for this high CEC value. The composition determined by X-ray fluorescence analysis calculated to a cation charge of +22 is K 0.68 (Mg 0.39 Al 1.02 Fe 0.54 3+ Fe 0.04 2+ )[Si 3.59 Al 0.38 Fe 0.03 3+ ]O 10 (OH) 2 . X-ray diffraction of glycolated and heated K-saturated Muloorina illite revealed no evidence of expandable layers. The cell dimensions are a = 5.314(1)Å, b = 9.040(1)Å, c = 10.135(3)Å, and b = 100.97(3)° (3σ in parentheses). Transmission electron microscopy revealed that Muloorina illite has a remarkably fine and uniform particle size in the form of irregularly stepped hexagonal crystals, averaging 60 nm in diameter at their widest, and 35 nm thick, each step being ~7 nm high. At most steps and between many steps defects occur where a 2:1 layer terminates, with a wide interlayer spacing beyond, commonly reaching to the crystal edge. Even though uninterrupted stacking of >10 or so 2:1 layers across one crystal is uncommon, the irregular distribution of the dislocations leaves appreciable structural continuity so that the mean diffracting thickness is of the order of 16 nm. Muloorina illite is not an interstratified illite-smectite, but a mica mineral with low-charge regions associated with terminating 2:1 layer defects. Such regions are wedge shaped at the nm scale and stabilized by the bonding of the illitic remainder of that interlayer. Approximately 17% of the total volume of each Muloorina illite crystallite is occupied by such low-charge 2:1 layers.