Quartz Crystallinity Index of Arabian Sands and Sandstones

Although quartz crystallinity index (QCI) was developed more than four decades ago, it remains a relatively new tool used in provenance studies. This study examines QCI values of Arabian sands (Quaternary sands from Nafud, Dahna, and AlRub'AlKhali deserts), and Arabian sandstones (Paleozoic and Mesozoic sandstones from northern and central Arabia). QCI values of sandstones range from 8.2 to 9.8. For Quaternary occurrences, QCI values of Nafud sands range from 7.4 to 9.5, Dahna sands range from 7.9 to 9.4, and AlRub'AlKhali sands range from 8.8 to 9.4. The data confirm that Arabian sands are derived mainly from early Paleozoic sandstones, with subordinate contributions from late Paleozoic and Mesozoic sandstones. They also highlight the role that sediment homogenization plays in narrowing QCI values of AlRub'AlKhali sands. Highest and lowest QCI values raise questions about the role of chert in controlling of QCI values of Arabian sands and sandstones. Arabian sandstones of Late Ordovician, Late Devonian, Late Permian, Late Triassic, and Cretaceous age possess the highest QCI values (>9.5). These time intervals coincide with lower abundances of siliceous sponges, and therefore fewer potential sources of chertification/silicification. Cambrian sandstones and Quaternary sands possess the lowest QCI values (<8.5). This coincides with the fact that both are of eolian origin, environments, which usually contain large quantities of quartz-rich dust that are documented to be a primary source of silica for chert formation. Therefore, we propose that the presence of chert is responsible for the extreme values of QCI in Arabian sands and sandstones.