Seeing Through Stone

Richard Fortey loves fossils. In Trilobite! he waxes poetic about the trilobite’s stone eyes.

Trilobite eyes are made of calcite. This makes them unique in the animal kingdom.

Calcite is one of the most abundant minerals. The white cliffs of Dover are calcite… Limestones (which are calcite) have been used to build… the sublime crescents of Bath, the pyramids of Gizeh, the amphitheatres and Corinthian columns of classical times. Polished slabs composed of calcite deck the doors of Renaissance churches in Italy, still grace the interiors of Hyatt–Regency hotels, or conference halls, or wherever architects wish to suggest the dignity that only real rock seems to confer.

The purest forms of calcite are transparent. In building stones and decorative slabs it is the impurities and fine crystal masses that provide the colour and design… The dark red of the scaglio rosso so typical of Italian church doors is a deep stain of ferric iron. But when a calcite crystal grows more slowly in nature, then it may acquire its perfect crystal form, and be glassy clear…

Look into a crystal of Iceland spar and you can see the secret of the trilobite’s vision. For trilobites used clear calcite crystals to make lenses in their eyes; in this they were unique. Other arthropods have mostly developed ‘soft’ eyes, the lenses made of cuticle similar to that constructing the rest of the body.

The science of the eye demands a little explanation. It all depends on the optical properties of calcite… If you break a large piece of crystalline calcite it will fracture in a fashion related to its fine atomic structure… You are left with a regular, six-sided chunk of the mineral in your hand, termed a rhomb… The clear calcite of this not-quite-a-cube treats light in a peculiar way. If a beam of light is shone at the sides of the rhomb it splits in two; this is known as double refraction. The rays of light so produced are the ‘ordinary’ and the ‘extraordinary’ rays: their course is determined, just like the shape of the rhomb, by the stacking of the individual atoms. There is a huge specimen of Iceland spar on the first floor of London’s Natural History Museum through which you may peer to see two images of a Maltese cross, one generated by the extraordinary, and the other by the ordinary rays. But there is one direction, and one direction only, in which light is not subjected to this optical splitting… from this direction it does not split into two rays at all but passes straight through.

If to travel back to the time of the trilobite is a historical sea-change then there can be nothing stranger than the calcareous eyes of the trilobite. And pearls are chemically the same as the trilobite’s unblinking lenses, being yet another manifestation of calcium carbonate, although pearls are exquisite reflectors of light rather than transmitters of it… The trilobite saw the submarine world with eyes tessellated into a mosaic of calcified lenses… his stony eyes read the world through the medium of the living rock.

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