Abstract
Alkali amphibole of rare compositions occurs as a rock-forming mineral in a high-Si phlogopite lamproite from Kvaløya, northern Norway. The amphibole typically occurs as small grains forming irregular and rosette-shaped aggregates in a matrix dominated by Fe-rich K-feldspar and quartz. Amphibole shows compositions ranging between the three limiting compositions: Composition C shows significant content of fluoro-potassic-magnesio-arfvedsonite, while composition A is a Fe2+, Fe3+ and CNa rich variety of potassic-obertiite. Composition B is characterized by an exceptional high value of C□. It is emphasized that the presence of C□ and CNa in amphibole needs to be confirmed by other methods.
The relationship between WO2−, C□,Ti4+ and Fe3+ of amphibole can be expressed by the following exchange operators, choosing potassic-magnesio-arfvedsonite [KNa2(Mg4Fe3+)Si8O22(OH)2] as the additive component: The two first exchange operators result in deprotonation of OH, while the two others result in the formation of vacancies on the C sites.
The presence of amphibole both in the lamproite and in the adjacent fenitized granite suggests that the mineral formed during reactions between rock and fluids derived from the volatile-rich lamproite magma. Possibly, amphibole core (composition A and B), formed in equilibrium with the fluid phase during crystallization of the melt, while amphibole rim (composition C) formed during subsequent mineral-fluid reactions. Presence of hematite in the lamproite matrix in addition to oxo-amphibole indicates that the rock formed during highly oxidizing conditions.
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