Vugs in late hydrothermal veins in the serpentinite at Gew-graze, Lizard, Cornwall, UK, contain serpentine spheres ≤0.7 mm in diameter composed of a crystallographically controlled radial array of well crystallized lizardite-1T crystals. Examinations with optical and scanning electron microscopy reveal that the spheres actually have polyhedral morphology. The polyhedral facets at the sphere surface are the (0001) terminations of individual single crystals of lizardite. Each lizardite crystal is a hexagonal prism and tapers inwards to the core. The angle from prism axis to prism axis is always ∼24°, and this angle is consistent even though individual prisms have not maintained contact during growth. The space between prisms is filled by smaller crystals of lizardite in more random orientations, forming a solid sphere. Collectively, the tapering prisms form a growth array that produces a surface tessellation consisting of mainly 6-fold neighbours, but with some 5-fold arrangements to accommodate a closed spherical structure. A ‘buckybalF, modified by adding face-centring points to each hexagon and pentagon, provides a useful model to describe the space filling adopted by the polyhedral lizardite spheres. Cross sections (close to an equatorial plane) through these polyhedral spheres resemble cross sections of polygonal serpentine, with 15 sectors at 24° to each other, though very much larger in diameter.

Twenty one samples of relatively pure tubular halloysites (HNTs) from localities in Australia, China, New Zealand, Scotland, Turkey and the USA have been investigated by X-ray diffraction (XRD), infrared spectroscopy (IR) and electron microscopy. The halloysites occur in cylindrical tubular forms with circular or elliptical cross sections and curved layers and also as prismatic tubular forms with polygonal cross sections and flat faces. Measurements of particle size indicate a range from 40 to 12,700 nm for tube lengths and from 20 to 600 nm for diameters. Size distributions are positively skewed with mean lengths ranging from 170 to 950 nm and mean diameters from 50 to 160 nm. Cylindrical tubes are systematically smaller than prismatic ones. Features related to order/ disorder in XRD patterns e.g. as measured by a ‘cylindrical/prismatic’ (CP) index and IR spectra as measured by an ‘OH-stretching band ratio’ are related to the proportions of cylindrical vs. prismatic tubes and correlated with other physical measurements such as specific surface area and cation exchange capacity. The relationships of size to geometric form, along with evidence for the existence of the prismatic form in the hydrated state and the same 2M1 stacking sequence irrespective of hydration state (i.e. 10 vs. 7 Å) or form, suggests that prismatic halloysites are the result of continued growth of cylindrical forms.