Fine-grained disseminated quartz in sedimentary rocks may often be characterized as being secondary in origin on the basis of its X-ray diffractogram. The existence of a diffractogram intensity ratio I 100 /I 101 substantially greater than that obtained from a random sample of crushed quartz appears to be a sufficient (but not necessary) criterion for recognizing quartz of secondary origin in the absence of a volcanic component. The intensity ratio anomaly reflects preferred orientation of quartz in the sample mount and results from the presence of faces on the (100) form (prism face) on some of the grains. These results are consistent with both geologic and oxygen isotopic evidence.

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