Original paper

Solid solution between potassic alkali amphiboles from the silica-rich Kvaløya lamproite, West Troms Basement Complex, northern Norway

Kullerud, Kåre; Zozulya, Dmitry; Erambert, Muriel; Ravna, Erling J. K.

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: A: A,B(K1.01Na1.99) C(Na0.26Mg1.58Mn0.03Fe2+ 0.91Fe3+ 1.6Ti0.470.13)TSi8O22 W[F0.97O1.03] B: A,B(KNa2)C(Na0.04Mg1.04Mn0.22Fe2+ 0.65Fe3+ 2.07Ti0.250.7) TSi8O22 W[F0.68Cl0.01O0.13(OH)1.18] C: A,B(K0.9Na2.1) C(Na0.04Mg3.54Mn0.02Fe2+ 0.28Fe3+ 1.04Ti0.030.02) TSi8O22 W[F1.34O0.06(OH)0.6] 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 [KNa2Mg4Fe3+)Si8O22(OH)2] as the additive component: Ti4+Mg2+ -1H+ -2 Fe3+Mg2+ -1H+ -1 Ti4+□Mg2+ -2 Fe3+ 2□Mg2+ -3 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.

Keywords

alkali amphibolefluoro-potassic-magnesio-arfvedsonitekvaløya lamproiteoxo-amphibolepotassic-obertiite