"Zircon and acidic volcanic glass present in the Toba rhyolitic tuff on Sumatra's east coast are characterized by a typical habit. Allanite, a mineral usually not occurring in acid volcanic material is also present. It is shown that allanite and zircon are concentrated in strongly weathered soils in such quantities that the total of the 'heavy' minerals (specific gravity 2.9) consists almost entirely of these two minerals."

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