Abstract

Phosphate minerals are frequent accessory minerals occurring in masses in the Malpensata pegmatite dike that crops out near the village of Olgiasca, Colico municipality, Lecco Province, Italy. Some of these masses show dendritic texture and consist mainly of Ca-rich graftonite-(Mn). In these masses the graftonite-(Mn) is often altered to secondary phosphate minerals and shows a rim of primary ferrowyllieite crystals. Electron microprobe analysis and X-ray powder diffraction were used to identify the following secondary phosphate minerals: vivianite, metaswitzerite, jahnsite-(CaMnMn), mitridatite, xanthoxenite, and fairfieldite. Petrographic textural analysis aided in the explanation of the complex alteration mechanism of graftonite. The first stage of alteration is marked by graftonite destabilization induced by intense hydration under non-oxidizing conditions with mobilization of Ca and producing the association vivianite + metaswitzerite. The second stage is characterized by oxidizing conditions and, thanks to the previous Ca mobilization, is marked by the crystallization of jahnsite-(CaMnMn), mitridatite, xanthoxenite, and fairfieldite at the expense of vivianite + metaswitzerite and some of the remnant of primary graftonite-(Mn). Furthermore, the crystallization of primary ferrowyllieite rimming the masses of graftonite-(Mn) is explained.

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