Tectonics & Paleo (4):The world of CPOs and ODFs

Materials in geoscience and biology are often not isotropic - their properties, such as conductivity, soundwave velocity, and shear strength, vary with direction.

The reason can be that they are crystalline - if so, the orientation of the crystal lettice of (one, a few, or) many individual crystal grains has an influence on the properties of the compound material. The individual crystal lettice orientations can be at random or their can be a preferred orientation.

A whole branch of mineralogy and material science deals with the analysis of materials which show crystal preferred orientations (CPOs).

From Tectonics...

Structural geologists, who are dealing with microscopic phenomena of tectonic deformation, use mineral textures of rocks, i.e. crystal preferred orientations of the rockforming minerals, as indicators of tectonic movement and deformational regimes (temperature, pressure conditions; deformation rate).

The so-called Orientation Distribution Density Function (ODF) describes how the crystal axes of mineral grains in a sample are oriented relative to an outer coordinate system (e.g. geographic XYZ coordinates). From the ODF, usely depicted as a couple of stereographic density plots (e.g. plots for the crystal a-axis, b-axis and c-axis orientations) the direction and sense of shearing of a rock can be inferred.

...to Paleontology

Not only tectonic forces are governing the crystalline properties of natural materials. In a similiar way the (often monomineralic) mineralized tissues of organisms are underlying specific biological formation conditions - the different layers of a skeleton can show rather perfect CPOs.

Crystal orientations in mineralized tissue are, of course, not defined relative to a geographic coordinate system but to the anatomic directions (or the axis of accretionary growth) of the animal as a reference system.

Coming from the tectonical side I was participating in a workshop on "Textures & Microstructures in Geosciences" in 2005: I was really surprised that there is an application of texture analysis in palaeontology and that some people are really doing it.

Methodology

You can do either single grain measurements or methods integrating all orientations of crystal grains in a certain volume of the sample.

U-stage and EBSD are involving single grain measurements of a thin section: For each grain in a certain raster the individual crystal orientation is determined - either by its optical properties using the universal stage (this old-fashioned manual method is rather time-consuming) or by the way how electrons from an electron microbeam are backscattered on a detection screen (Electron Backscatter Diffraction, EBSD). The detected Kikuchi line patterns are indicative for how an individual crystal is oriented (they can be interpreted automatically). What you get in both approaches is not only an ODF but also a crystal orientation map of your thin section.

XRD, Neutron Diffraction. In these methods an X-ray or neutron beam is used to measure a larger volume of a sample comprising several crystals, thereby neutron radiation can penetrate even larger samples completely while the X-ray has a relatively low depth of penetration. From the detected line sprectra pole figures, representing the distributions of certain crystal lettice plane orientations can be derived (from which in turn the ODF of all the crystals in the measured sample volume can be deduced).

Examples from Paleontology

Chateigner, D., Hedegaard, C. & Wenk, H.-R. (2000): Mollusc shell microstructures and crystallographic textures. - Journal of Structural Geology 22: 1723-1735.

The authors employ X-ray diffraction measurements and demonstrate thate the microstructures and crystallographic textures of aragonite layers of species from different mollusc taxa including bivalves, cephalopods, gastropods and monoplacophorans are highly specific and contain a phylogenetic signal: closer relatives are more similar in their shell's crystal orientations.

Pyzalla, A.R., Sander, P.M., Hansen, A., Ferreyo, R., Yi, S.-B., Stempniewicz, M. & Brokmeier, H.-G. (2006): Texture analysis of Sauropod bones from Tendaguru. - Material Science and Engineering A 437: 2-9.

This neutron diffraction approach addresses the question whether the apatite crystallite textures in adolescent and adult Brachiosaurus long bones show some signal indicative for specialized crystal orientations which can be attributed to the giant growth of sauropod dinosaurs. However, comparing their results to the measurements of turkey and other dinosaur long bones they found no significant difference in texture strength or in the predominant direction of fibres.

Prospects

Given the elaborateness of most approaches, measurements of crystallographic textures are rarely used in paleontology - I suppose this will change if it turns out that the analysis of the crystal orientations can provide substantial information which is not obtained from the usual analysis of skeletal histology and microstructures.