So far we have taken a fairly qualitative approach to measuring microstructures, however if you actually want to compare microstructures from different areas it is much better to have an accurate measure of, for example grain size distribution or modal analysis. The main problem with this is that it is really boring to do. In this exercise we will divide the class up into 4 groups and each group will measure the grain size distribution, grain shape foliation orientation and aspect ratio of one of the four quartzites you looked at in the last lab.

In order to do this we have used the video microscope to capture "grey scale" images of the thin sections, and you will use the image analysis package called "NIH-Image" to perform this analysis.

Grain Size, Grain Shape, Grain Shape Foliation

These parameters are often measured through pretty laborious manual techniques, and assuming that you are interested in the analysis of differences in values, then the following technique is not too time consuming...

The basic steps of the analysis are:

0) Open up the file 16xqtz.pict using the Open option from the File menu, this is a photo-micrograph of a deformed quartzite.

1) Trace boundaries of all the grains using the tool, making all the boundaries white.

2) Turn the image into a thresholded image: select Threshold from the Special menu. This will duplicate the image and convert it to black and white.

3) Using the bucket tool, fill the outside areas with white.

4) Analyse the grains outlined in the new image: select Analyse from the Special menu. This will also open up a new window with all the grain size information for each grain.

5) Save this text information out to file by using the Save option from the File menu.

6) Read in this new text file from Cricket Graph, and make frequency plots (I'll tell you how in the class). 

Modal Analysis

For certain rock types, such as this gabbro, it is possible to do a semi-automated modal analysis instead of point counting, and these techniques work even better if you have colour images.

1) Load in gabbro.pict file. In this image the dark grains are magnetite, the grey grains are amphiboles, and the light grains are feldspars.

2) Threshold the image to individually highlight the magnetite, amphibole & feldspar domains.

For each thresholded image create a histogram, which will show the number of pixels of black and white, and hence the proportion of that mineral which is present in the image. Notice the problems that arise with respect to alteration along cracks.

Lab 3c- Foliation development in naturally deformed rocks

73-0711 Foliated Greywacke This thin section demonstrates a common problem in low grade metamorphic terrains: how to distinguish sedimentary foliations from metamorphic/structural foliations.

• Make a sketch of any features which can be used to show whether this rock has undergone any deformation at all.

Lake Glenmaggie slide This thin section also shows at least two strong overprinting relationships. Sketch the relationship of the foliations present in this rock, and describe the processes which appear to have been involved in the formation of each foliation. Notice the strong lithological control on foliation development.

Make a composite sketch showing the relationships between each foliation, and show the trace of the earlier foliations as they get reoriented by later events.