CRYSTALLIZATION OF VARISCITE AT ROOM TEMPERATURE

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Abstract
Aluminum phosphate precipitates were prepared by mixing AlCl3, NaH2PO4, and NaOH in various combinations and were periodically characterized for up to at least 4 yr. In samples of low Al (0.0005 M) and low phosphate (P/Al molar ratio = 0.3, 0.6 or 0.9) concentrations, with pH 4 and above, the precipitates were amorphous basic aluminum phosphates that did not indicate any tendency toward crystallization to variscite during aging up to at least 55 mo. In samples 0.05 M in Al and 0.4 M in NaH2PO4 and NaOH/Al = 0 to 1.5, an unidentified aluminum hydrogen phosphate began to appear 6 mo. after preparation, which later slowly transformed into variscite after prolonged aging. The reaction products were pure variscite after prolonged aging. The reaction products were pure variscite with NaOH/Al = 0 (pH 2.6) after 71 mo. and with NaOH/Al = 0.5 or 1.0 (pH 2.7-2.9) after 45 mo. With NaOH/Al = 1.5 (pH 3.3), the product consisted of 20% variscite and 80% of amorphous material at the end of 45 mo. With NaOH/Al = 2.0 and above, the products remained amorphous to X-ray diffraction and rapidly dissolved in Mehlich''s solution (0.05 N HCl-0.025 N H2SO4) even after 5 yr of aging. That variscite was developed only under very restricted conditions suggests that this compound might not be the end product of phosphate fertilization in acidic soils.

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