Using standard hydrothermal techniques, the phase relations of three Al-phosphate minerals—berlinite, AlP04; augelite, Al2P04(0H)3; and trolleite, Al4(P04)3(0H)3—were determined in the temperature and pressure range of 400° to 600°C and 500 to 3000 bars. The univariant line, defining the upper stability limit of trolleite (trolleite = augelite + berlinite), can be expressed by T(°C) = 0.0373P(bars) + 350.7. The univariant curve defining the upper stability of augelite (augelite = berlinite + corundum + vapor) is expressed by log P(bars) = —14,938/T (°K) + 22.48.

These curves intersect at an invariant point near 520°C and 4500 bars. The addition of diaspore leads to another invariant point at approximately 550°C and 12 kilobars. The complete P-T diagram indicates that the stability field of trolleite is greatly extended by pressure, whereas the stability of augelite abruptly terminates near 520° C, regardless of the pressure.

Calculations, based on the breakdown of augelite, indicate that excess quartz reduces the stability of augelite by about 35°. From the calculated free energy of augelite (ΔG = —671.5 ± 1.5 kcal at 25°C) it is concluded that Al-phosphate minerals in the presence of phosphoric acid are far more stable than any Al-silicate. This means that if Al-silicate minerals have formed by hydrolysis of micas and feldspars and if the hydrolyzing fluid contains any of an H3PO4 component, Al-phosphate minerals will also form.

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