The Early Eocene Fur Formation is a clayey diatomite interbedded with layers of volcanic ash. The diatomite is non-calcareous, laminated or structureless, and thin sections show that the proportion of clay-sized particles to diatom frustules varies between laminae. The clay mineral phase has been isolated and its chemical composition has been determined by X-ray fluorescence (XRF). Clay mineralogical analyses based on X-ray diffraction (XRD) show that the dominant phase is smectite. A general formula for the clay mineral phase is: X0.1K0.2(Mg0.3Fe(III)0.3–0.7Al1.1–1.4)Si4O10(OH)2.The investigation indicates that the clayey diatomite of the Fur Formation comprises three components: diatom frustules, clay minerals and volcanic dust. The chemical compositions of bulk samples have been determined. The clay mineral content is calculated to be 30–45% (by weight). Volcanic dust constitutes c. 10% of the diatomite, and the rest is diatoms (opal-A). The diatom frustules contain some Al2O3 that is either of primary or diagenetic origin.