Accessory allanitc was separated from phanerocrystalline igneous rocks and its optical properties and radioactive components were compared. The indices of refraction of these allanite samples are higher than those from the pegmatites that are usually described in geologic literature. The birefringence was found to range from 0.015 to 0.021, the α-index of refraction from 1.690 to 1.775. The allanite content ranges from 0.005 to 0.25 percent by weight in the rocks studied. The mineral is confined largely to the more siliceous phanerites. The uranium content is highest in the allanite from the granites sampled, ranging from 0.004 to 0.066 per cent, whereas the thorium content is high or low regionally, ranging from 0.35 to 2.33 per cent. Allanite was found to be otherwise of exceptionally uniform composition.

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