Quartz as an internal standard for identification of minerals

Frequently, when using X-ray diffraction to identify minerals in a powdered rock sample, there is a need for a calibration standard (e.g. sil icon) to el iminate systematic errors in recorded posit ions of diffraction peaks. However, adding another component to a mult imineral sample is not desirable as the consequent dilution reduces the intensity of diffraction peaks to be identified and increases the possibi l i ty of masking. The diffraction patterns of six different variet ies of quartz (SiO2) were measured and compared to determine if the quartz natural ly present in a rock can be used as an internal standard for measuring the 20 posit ions of X-ray diffraction l ines of other minerals in the same rock, and thus el iminate the need for adding a standard (unit-cell d imensions were calculated by a least-squares computer program). The study indicates that probably all well crystal l ized quartz can be used for this purpose (with 20 posit ions as listed under 'avg obs' in Table 1) provided only moderate accuracy is required (0.010.02 ° 20). The author is current ly using natural ly present quartz as an internal standard for cal ibrat ing the whole rock diffraction patterns of some si l icif ied Archean andesitic tufts from the pre-Cambrian Shield of Canada (these rocks are composed of quartz, plagioclase, chlorite, sericite, and actinolite).