Published data indicate that in igneous rocks the delta 18 O of quartz averages about +9 per thousand ; in metamorphic rocks, +13 to +14 per thousand ; in sandstones, +11 per thousand ; in shales, +19 per thousand ; quartz overgrowths in sandstones, +20 per thousand ; and cherts, +28 per thousand . Either a very large proportion of the silt and clay-size quartz in shales (about 90% of the total quartz in shales) is secondary, or the isotopic data are incomplete. It is concluded that the latter interpretation is correct because published isotopic analyses of metamorphic rocks consider only the coarser-grained rocks. Phyllites and slates, whose quartz is nearly all of silt and clay size and has more positive delta 18 O values than quartz from schists and gneisses, probably supplies the bulk of quartz to mudrocks.

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