EES Group Pages
The Puzzle of Unexpected Disorder: Probing Rocks with Neutrons
Neutrons give Earth scientists a unique opportunity to look at the structure of rocks in ways never before imagined. Neutrons can probe the atomic structures and dynamics of ordered and disordered materials such as crystals and glasses (e.g., quartz and obsidian), and combinations of the two. Because they have no charge and scatter only from atomic nuclei, neutrons can penetrate deeply into most materials, including intact rocks. This is important. The grains and bonds at the surface of a rock behave differently from the interior grains because they are exposed to air and humidity and they are not surrounded by other grains. X-rays, a competing way to look at atomic structure, do not scatter effectively from lighter elements like Calcium and Silicon (some of the most abundant elements on Earth) nor can they be readily used on intact samples.
The physics of why rocks behave the way they do is still not well understood. Rocks are peculiar granular materials; they are nonlinear (i.e., not perfect "springs") and hysteretic (changes of length lag changes in force). They also display some unusual memory effects (see "The Memory of Rocks" Physics Today, April 1999). For example, an acoustic wave traveling through a rock can soften the rock. This softness goes away only gradually; the rock can take from several hours to a day to recover (a memory effect). Rocks are some of the easiest and most accessible granular materials to study and their peculiar behavior can easily be measured. Moreover, rocks are of great interest to Los Alamos as a part of a larger class of granular materials, varying from oil- and gas-bearing rocks to concretes to high explosives.
Thin section (polarized) of a pure Quartz sandstone. Careful examination shows no amorphous/glass present.
If all or some of the atoms are not in an ordered array like found in a crystal, Pair Distribution Function (PDF) analysis of the neutron data can tell us average local distances between atoms – suitable for analyzing glasses or liquids or disordered materials in general. We decided to use the Lujan Center's NPDF beamline at LANSCE to look for disordered/non-crystalline/glassy bits in pure quartz sandstones. The NPDF beamline has been upgraded from a high resolution powder diffraction line to a specialist PDF machine for local structure studies and is arguably the best machine in the world for such a study.
PDF plot (atom-pair-separation distance along the bottom axis). The data are red, the model (pure crystal) blue. Note the excess intensity measured in the first two peaks that are NOT part of the crystalline model. These peaks are related to the distance between Si and O atoms and O and O atoms which are not part of the general crystal structure of the grains making up the rock.
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Funding Acknowledgments: LDRD-ER, NSF (NPDF beamline upgrade), NASA (summer students), OBES (JT)