Abstract

Lizardite-1T crystals from Gew-graze at the Lizard, Cornwall, of an apparent composition (Mg2.94Fe0.03Al0.03)(Si1.97Al0.03)O5(OH)4, have been observed to contain 2 – 5% of thin interstratifications of clinochlore when investigated by analytical transmission electron microscopy (TEM). The clinochlore is usually only a few unit layers thick but has an extensive lateral dimension parallel to the lizardite layers. Analytical TEM confirms the existence of very low-Al lizardite with a planar structure interstratified with occasional thin units of clinochlore containing substantial Al. Compositionally, such clinochlore (invisible by X-ray diffraction) interstratified throughout the lizardite crystals, could account for most of the Al present. We suggest that an occasional influx of Al causes the nucleation and growth of clinochlore at the expense of lizardite. The possibility of clinochlore contributing to the measured Al content in lizardite samples highlights the need for future investigations of Al-bearing serpentines to include careful examination and interpretation using imaging and analysis by TEM.

It has crept into serpentine discussions that lizardite cannot form without the coupled substitution of Al to relieve the misfit between the sheets of octahedra and tetrahedra. The lizardite at Gew-graze is almost Al-free, forms well-crystallized planar crystals and demonstrates that Al is not an essential element for lizardite crystallization.

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