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The Hawaiian beaches are predominantly calcareous sands of skeletal origin washed in across the reef flats. Calcareous deposits also are present in the island shelf and bank environments. The carbonate minerals listed in order of decreasing abundance are high-Mg calcite, aragonite, and low-Mg calcite. Relative abundance of these carbonate mineral assemblages was determined by X-ray diffraction analyses to correlate the mineralogy, the sources, and the depositional environments. Beaches of the island of Hawaii, the youngest of the Hawaiian Island chain, have a higher aragonite content (65%) than those of the other islands, and the older the island, the lower is the aragonite content of its beach sands. This phenomenon is probably due to the maturity of the reefs. Coral and Halimeda, being the first to develop, formed the aragonitic framework which gradually was filled by high-Mg calcite skeletal sands, and, locally by low-Mg calcite skeletal sands.
Aragonite is most abundant in the upper part of the island shelf (50 m) and high-Mg calcite increases in abundance in the lower part of the island shelf. Low-Mg calcite makes up less than 20% of the island shelf and is uniformly distributed in it. Aragonite in the lower part of the island shelf is transported from the shallow part of the shelf. The abundance of high-Mg calcite and aragonite in Hawaiian marine sediments indicates the recentness of the calcareous deposits.
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