Fabric Analyser - Investigator

AVA maps

Images are typically in the range of 1000 x 1000 pixels (single analysis, 5 um resolution) and 7000 x 5000 (whole thin section, 5 um resolution) and reprocessed as axial-distribution diagrams or AVA diagrams (Achsenverteilungsanalysis) or as geometric quality or retardation maps (see section 'About FA&I') with scatter diagrams of individual grain c-axis orientation plotted on an equal area sterographic net. A standard colour code is used to identify the c-axis directions as azimuths and dips in the micrographs. In this scheme, north trending axes are in blue, east trending axes are in orange, axes normal to the plane of the thin section are in black and gradational shades represent angular differences between neighbouring pixels. A change in colour intensity to the centre of the colour circle (black) corresponds to c-axes increasing in plunge from horizontal, on the periphery, to vertical at the centre of the net. This is an easy method of measuring grain orientations and large data-sets can be collected quickly (e.g. in the above analysis of 600 grains, these single points were measured and plotted in 30 minutes; the capture of the images used to make the AVA took 5 minutes).

Download the latest INVESTIGATOR and the manual.

Geometric quality maps

The fabric analyser establishes the azimuth of the extinction plane for the c-axis and the light direction. There are eight oblique planes and one vertical plane. The geometric quality is a measure of how tightly these planes intersect. Ideally, it would be a single line - the c-axis direction. In practice, there are multiple interections. The general algorithm for determining the c-axis is to accept that the vertical axis establishes the azimuth and that the outer most-inclined extinction planes establish the colatitude plane. The colatitude is estimated from the mean of these two intersections of the outer oblique planes with the vertical plane. The geometric quality is then established from the sum of deviations of the azimuths of the eight oblique planes from the azimuths of the ideal planes through the light direction and the established c-axis direction.

The measured colatitude for near horizontal c-axes is sensitive to small azimuthal errors, and the measured colatitude for near vertical c-axes is less sensitive to azimuthal errors. The geometric quality takes this colatitude sensitivity into account.

Retardation quality maps

The retardation quality is a measure of how well the phase values of red, green, and blue light match. In the fabric analyser, the phase values are generated from the ratios of the plane polarized light and the plane polarized light with a 550nm mica waveplate. The section retardation value is given by the phase angle plus an integral number of wavelengths. The green wavelength count determines the order of the interference colour in Newton's scale.

               blue retardation = (blue phase + i) * blue wavelength
              green retardation = (green phase + j) * green wavelength
                red retardation = (red phase + k) * red wavelength
               
               where the i,j,k are integers with i>=j>=k.

After the appropriate values of i,j and k have been selected to maximise the agreement of the three retardation values, the retardation error is given by the sum of the differences between the green retardation and the blue and red retardations expressed as phase errors.

Use of INVESTIGATOR software

The process yields spatial information describing individual grains and their orientation within a sample. Each c-axis is recorded with its XY position, orientation and relationship within the captured sequence. The results can then be analysed using a variety of discriminators to sort and select different collections of candidate grains to prepare fabric diagrams or analyse nearest neighbour grain relationships.

You will be presented with the following files to work with: azimuth bmp i.e 360 colour wheel and colatitude bmp colatitude bmp geometric quality map bmp information file e.g. mineral analysed, image data crystal glass bmp -identifies holes in section orientation bmp overall quality bmp retardation bmp trend flat i.e. 180 azimuth of grains trend bmp i.e. trend with colatitude 8x cross polariser images Areas where c-axes are required are selected using a cursor Clicking on an individual grain identifies the plunge of the individual c-axis Data can be saved as EXCEL file and as a cumulative stereonet Misorientation relationships can be analysed to calculate neighbour-pair misorientation from c-axes on either side of a grain boundary. Random-pair misorientation relationships are calculated from the existing neighbour-pair datas using EXCEL macros.