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Polyhedral serpentine: a spherical analogue of polygonal serpentine?

¹ Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK
² Electron Microscopy Centre, School of Chemistry, University of Southampton, Southampton SO17 1BJ, UK
³ Natural History Department, Royal Ontario Museum, 100 Queen's Park, Toronto M5S 2C6, Canada

^* E-mail: g.cressey{at}nhm.ac.uk

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 `buckyball', 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.

KEYWORDS: serpentine, polyhedral, polygonal, Lizard, UK