The textural analysis is a powerful, but underused tool of petrostructural analysis. Except acquirement of common statistical parameters, this technique can significantly improve understanding of processes of grain nucleation and grain growth, can bring insights on the role of surface energies or quantify duration of metamorphic and magmatic cooling events as long as appropriate thermodynamical data for studied mineral exist. This technique also allows systematic evaluation of degree of preferred orientations of grain boundaries in conjunction with their frequencies. This may help to better understand the mobility of grain boundaries and precipitations or removal of different mineral phases.
We introduce a new open platform, object-oriented MATLAB® toolbox PolyLX providing several core routines for data exchange, visualization and analysis of microstructural data, which can be run on any platform supported by MATLAB®. Detailed descriptions of toolbox routines and methods of implementation of new techniques are given in PolyLX Reference Manual.
The quantitative textural analysis rely on detailed and precise description of grain sizes, grain shapes, grain boundaries as well as preferred orientations of grain and grain boundaries. The manual or semi-automatic digitalizing of scanned or digital photographs could be done in any software which is capable to export ESRI Shapefiles. The desktop GIS packages which allow create topologically correct geometries are best candidates. The grains are generated in form of adjacent polygons and each grain is labelled with a unique ID and a phase name. The unique ID is used to keep relations integrity with imported results of analysis, when visualization capabilities of GIS packages are in demand. Once all grains are digitized, PolyLX toolbox can build a set of polylines representing individual grain boundaries. Individual boundaries span from neighbouring triple points and are labelled by a unique ID and by ID’s and phase names of adjacent grains. For those having access to ESRI ArcView 3.x, the available PolyLX extension could be used to generate boundary files and perform some error-checking tasks.
The grain and/or grain boundary geometries together can be directly imported from ESRI ArcView shapefiles. In MATLAB® environment grain and grain boundaries are represented as objects, which hold all important properties, such as ID, Phase name, Area, Perimeter, Axial ration, Length, Width etc. All these values can be obtained on-the-fly and statistically analysed. Along with wide spectrum of mathematical and statistical methods of data exploration available in MATLAB™, PolyLX incorporates several geologically specific techniques:
Shape is extremely difficult property to measure, or even to define in a precise manner. Perhaps this is why there are so many proposed shape measures, none of which has been proved as entirely satisfactory. A shape measure should possess several desirable properties. Obviously, objects with different shapes should yield different measures, and similarly shapes should yield similar values regardless of the size or orientation of the object. Unfortunately, a shape measure possessing these properties may be a chimera; it has been prove that no single measure can be unique to only one shape. Therefore, there is a wide spectrum of single value measuring methods available in PolyLX toolbox.
To obtain data of preferred orientation, several techniques are implemented in PolyLX toolbox. The most general one is method of analysis matrix of inertia. This method can be applied on individual grains or boundaries as well as on a set of grains or grain boundaries. In latter case, the result is weighted by size of objects, which is welcome in specific tasks and differs from the results obtained from orientation analysis based on histograms (rose diagrams) or Fisher distribution. Another group of routines using approach of direction dependent projection of grain or grain boundaries (PAROR, SURFOR and PARIS) and advertised by Pannozo (1983) are fully implemented.
One of the most important aspects of quantitative texture analysis is description of spatial characteristics of grains or grain boundaries. PolyLX contains several routines dealing with spatial distribution of grains or grain boundaries (grain density method, nearest neighbour analysis, spatial pattern index) or evaluating deviation from random distribution (contact frequency and contact area methods). Crystal size distributions: PolyLX offers implementation of method to construct the CSD plots using technique described by Peterson (1996).
Several techniques to estimate strain are available. Classical ones as Rf/φ, centre-to-centre method and Harvey & Fergusson (1981) as well as some of their recent modifications like DTNNM (Mulchrone, 2000) or area weighted Rf/φ are implemented.
You can download my short presentation of PolyLX toolbox here.
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