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Local atomic structure of Fontainebleau sandstone: Evidence for an amorphous phase?

Rocks are traditionally viewed as crystallite aggregates; in general the treatment of the composition and mechanical properties never includes solid non‐crystalline components. Such glass‐like materials are difficult to detect in small quantities by standard techniques (e.g., thin‐section polarimetry, x‐ray diffraction) but if they exist at critical locations (i.e. grain contacts) they could affect the behavior of rocks considerably. Neutron scattering measurements on a solid sample of Fontainebleau sandstone have shown clear evidence for the presence of an unexpected glass‐like component. Atomic pair distribution function (PDF) analysis shows significant local structural deviations from pure quartz. These deviations appear as an excess of 5–10% of nearest neighbor (NN) Si‐O and O‐O bonds, which is consistent with a 5–10% volume fraction of vitreous silica. These measurements may provide significant information about the still‐unexplained causes for the peculiar mechanics and dynamics of sedimentary rocks.