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What is the connection between a virus and a crystal? Well, sometimes a virus is a crystal. They can crystallise and (as far as I know) still be infectious afterwards. Indeed, biological crystallography is based on the idea that proteins crystallise (and diffract) just like minerals. So what's the difference between a protein crystal and a salt crystal?

Firstly, the basic repeating unit of salt is atomic wheras the protein is molecular (thousands to hundreds of thousands of atoms). Secondly, the protein forms a very much more complex (difficult to describe) shape. Thirdly, protein structure is encoded in the virus DNA and can be changed via changes to the coded information. Just as air-conditioning systems have blueprints, while marble floors do not; so more information is required for one structure than another.

So is it at all possible to have mineral organisms? Well, there is (at least) one feasible hypothesis that involves quite complex inorganic crystals and mechanisms for their replication and selection. This mineral is clay: or structures like it. Clays form a heterogeneous group of crystal forms that are relatively sensitive to environmental conditions for their growth. They also 'breed' by breaking in half to form two new crystals. Their shapes are heritable, in the sense that the grain pattern templates new layers with the same pattern. Like stacking carpets according to the direction of the arrows woven into them.

So is this life? Well, it doesn't eat, excrete, reproduce etc like organic life but it's a close approximation. More importantly, many industrial catalysts are mineral. Inorganic surfaces provide good reaction sites for organic reactions. If these surfaces are selected and reproduced on the basis of their catalytic efficiency then they provide a possible stepping stone to the life we see around us today....

A better description is provided by A. G.Cairns-Smith in his book 'Seven Clues to the Origin of Life'.

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