Crystallinity Index Standards (CIS)

Table 1. CIS values equivalent to the measurement scale of Warr & Rice (1994).

Name Metamorphic grade Locality
Kübler Index
Árkai Index
SW1-1992 Diagenetic
Widemouth Bay
0.630
-
SW1-2012 Diagenetic Widemouth Bay
0.654
-
SW2-2012 Lower anchizone Crackington Haven
0.452
-
SW3-2000 Lower anchizone Widemouth Bay
0.470
-
SW3-2012 Upper anchizone Polzeath Beach
0.367
0.295
SW4-2012 Upper anchizone Portgaverne
0.355
0.310
SW5-2000 Upper anchizone Strangles Beach
0.360
0.300
SW6-2012 Epizone Trebarwith Strand
0.260
0.257
SW7-2012 Epizone Delabole slate quarry
0.270
0.261

 

 


 

How to calibrate

Step 1. Wash the standards in deionised water prior to preparation to remove any natural salts remaining in the rock chips. Best to exchange the water several times. The presence of any salts can be tested using silver nitrate solution.

Step 2. Once washed, remove any weathered fragments or fragments that are strongly strained with iron oxides/hydroxide precipates (coloured brown). The remaining fresh rock chips are now ready for preparation following your standard procedure.

Step 3. Analyse the standards following your normal analytical procedure. They should be treated in exactly the same way as you sample material.

Step 4. Measure the Full-Width-at-Half-Maximum (FWHM) values of the 10 Å (001 illite-) and 7 Å basal (002 chlorite) reflections, known as the Kübler and Árkai indices. You can do this using standard "point peak" methods or by profile fitting. In the case of profile fitting you will need to use single, asymmetrical shapes (e.g. Split Pearson VII) for the purpose of calibration.

Step 5. Plot the experimental FWHM values you have measured against the CIS values given in Table 1 in an X-Y plot.

Step 6. Fit a regression curve to the data set. It represents the difference between your experimental FWHM values and the given standard values. The correlation coefficient represents the goodness-of-fit and the quality of the calibration curve.

Step 7. The equation of your regression curve can then be used to convert the experimental values into calibrated values using standard spread sheet programs. I recommend the regression curve details be included in your publications.

A example of a calibration curve is given below.

 

 

 

Economic geology and mineralogy research group
University of Greifswald
Institute for Geography and Geology
Friedrich-Ludwig-Jahn-Str. 17A
D-17487 Greifswald
Germany

Contact
L.N. Warr. Tel. ++49(0)3834 86 4578. E-mail: warr@uni-greifswald.de.

 

Additional information

References