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)TSi8O22W[F0.97O1.03]B:A,B(KNa2)C(Na0.04Mg1.04Mn0.22Fe2+0.65Fe3+2.07Ti0.250.7)TSi8O22W[F0.68Cl0.01O0.13(OH)1.18]C:A,B(K0.9Na2.1)C(Na0.04Mg3.54Mn0.02Fe2+0.28Fe3+1.04Ti0.030.02)TSi8O22W[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 [KNa2(Mg4Fe3+)Si8O22(OH)2] as the additive component:  
Ti4+Mg2+1H+2Fe3+Mg2+1H+1Ti4+Mg2+2Fe3+2Mg2+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.

You do not currently have access to this article.