The decarbonation of carbonate-fluorapatite (francolite) has been studied in the range 599°–747°C, at atmospheric pressure. Progressive C02 loss from the apatite is characterized by color change, an increase in crystallinity, and the appearance of an anomalous υ2 doublet in the infrared absorption spectrum. Interpretation of the rate data with diffusion theory gives an activation energy of 62.5 kcal/mole and entropy of activation within the limits 3.8 to 11.2 cal/mole/deg. Significant deviations from ideal kinetics occur at high fractional C02 losses. Application of the data to the thermometry of the ‘Mottled Zone’ (Israel) thermal metamorphism gives 300°C as a minimum temperature estimate for the event. Optimum conditions are suggested for industrial processes which concentrate phosphorites by roasting.

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