Abstract

Aluminum orthoposphate (AlPO4) has polymorphs isostructural with tridymite, cristobalite, and quartz. Berlinite is the low-temperature form that corresponds to α-quartz. We report berlinite-quartz solid solutions to crystallize from a synthetic P-rich peraluminous granitic melt, similar in composition to the most volatile-rich silicate melt inclusions found in pegmatites. The crystallization took place in experiments performed in cold-seal pressure vessels at 450–700 °C and 0.1–0.2 GPa H2O pressure. At these conditions, the berlinite-quartz mutual solubility is limited to 5–7 mol% SiO2 on the phosphate side of the solvus and to the maximum of 1 mol% AlPO4 on the silica-rich side. The mutual solubility appears to decrease with falling temperature. At low T the crystals of berlinite-quartz solid solutions are strongly zoned and show complex intergrowths between the P-rich and silica-rich phases. They were studied by electron microprobe, transmission electron microscopy, and Raman spectroscopy. In the light of our new experimental results, the extreme P enrichment reported earlier for some natural quartz-hosted melt inclusions may be explained as a post-entrapment contamination by the berlinite-bearing host.

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