A study of zircons in sedimentary rocks

Lately we have ascertained the characteristic features of zircon in igneous rocks. In that case we have also pointed out that the zircons of probable sedimentary origin were encountered in igneous rocks. In this paper, the writers attempted to reveal the mineralogical features of zircon in sedimentary rocks, mostly sandstones of Tertiary age from Northern Kyushu and Hokkaido districts. Many varieties of zircon with such color as pale orange, pale blue, brown, purple and colorless are observed in these sedimentary rocks. These zircons are usually 0.15×0.06mm in size and have prominent ‘c-type’ crystal phase. And it should be emphasized that these sedimentary zircons presented characteristically rounded ‘R’ form. Distinction between sedimentary zircon and igneous zircon would be clear from the fact that zircons of rounded form ‘R’ amounted to more than 20 to 50 per cent in the sedimentary rocks in contrast with less than 20 per cent in igneous rocks.