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Specimens of palygorskite from Warren Quarry, Enderby, Leicestershire, have been studied in the laboratories. The mineral is found in dioritic rocks in association with angular dolomitic rock fragments as the filling material of a joint, the walls of which are sometimes lined with calcite and dolomite. Palygorskite occurs as 3-10 mm long bundles of fibres, 0·03-0·1μm wide. X-ray diffraction data indicate that the mineral may occur in the monoclinic form. The structural formula for the half-unit cell is Si8(Al1.89Fe0.053+Ti0.01Mg1.99)O20(OH)2(OH2)43·61 H2O.Ca0.07. Data obtained from differential thermal and infrared analyses are comparable with those for palygorskite in the literature, but the six-sided transversal cross-section of the fibres as revealed by electron microscopy is unexpected. The mineral was formed after the deposition of the overlying basal Triassic rocks when ground waters penetrated cracks in the dioritic rocks, causing Si and Mg to be released there and above the unconformity.

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Abstraet--A semi-micro quantity (10-15 mg) sample of clay is pressed into a 3 mm diameter pellet for differential thermal analysis (DTA) in a STONE-TRACOR ring microsample holder. The pellet is transferred to the sample holder dish with a vacuum tweezers. Platinum sample dishes are also handled by vacuum tweezers to avoid denting the dishes or damaging the thermocouple wires. The DTA curves obtained by the pellet method are highly reproducible. The resolution and sensitivity of reaction peaks are equivalent to or better than those produced by a large sample (200 mg) packed into a nickel block holder.