In an experiment, Mark Fox-Powell froze hydrothermal fluids and looked under a scanning electron microscope at the deposits that resulted. This net-like material is cryogenic opal-A -- silica-rich material that formed in the pore spaces between freezing grains of ice.
Detailed electron microscope views from Mark Fox-Powell's experiments. Upper left: a close-up on the net-like morphology of the cryogenic opal-A. Upper center: a typical broken piece of opal-A net. The rest of the pictures show examples of microfossils, the results of microbes being entombed in the opal-A as it forms. Some microbes are preserved, others appear only as voids.