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The definition of ‘clay’ was first formalized in 1546 by Agricola. It has been revised many times since, although the fundamentals involving plasticity, particle size, and hardening on firing were retained by most. For an exhaustive account of the history of the definition to 1963, the reader is referred to Mackenzie (1963). More recent developments may be found in Weaver (1989).The definition of clay raises the important issue of clay constituents and, implicitly, the definition of ‘clay mineral'. Mackenzie (1963, p. 15) noted the inappropriateness of defining clay mineral as “any mineral which occurs in clay” since, among several reasons, it would include many accessory minerals that are not characteristic of clay.

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Lanthanite-(Nd) occurs in tuffaceous, altered andesitic agglomerate in the Whitianga quarry, Coromandel Peninsula, New Zealand. The usual platy habit is represented, together with more unusual blocky equant crystals. The former show the new forms {201}, {102} and {111}, and the latter, {111}. ICP-MS analyses show that the distribution of REE (and Ga) is consistent with the formulation (Nd 0.63 La 0.59 Ce 0.35 Pr 0.15 Sm 0.10 Gd 0.069 Y 0.06 Eu 0.03 Dy 0.02 Ga 0.01) ⌺2.04 (CO 3) 3 •8H 2 O; the REE sum of 2.04 is due solely to rounding errors. The Þ nd represents the Þ rst occurrence of this rare mineral in New Zealand. The mineral formed under comparatively oxidizing conditions. The REE are probably scavenged by warm waters circulating through underlying greywackes of the Manaia Hill Group. We also characterized the lanthanite-(Nd) by X-ray diffraction (powder method) and Raman spectroscopy.