Mn- and F-bearing rasvumite in natrocarbonatite at Oldoinyo Lengai volcano, Tanzania

Abstract

Abstract Several K-Fe sulphide minerals have been described from natrocarbonatite lavas erupted from the Tanzanian volcano Oldoinyo Lengai but uncertainty remains about their exact identity. They do not appear to be any of the established K-Fe sulphides according to Dawson et al. (1995) and Mitchell (1997). Here, we describe yet another variant which is a Mn- and F-bearing variety of rasvumite. It appears to have formed by reaction of Fe alabandite grains with natrocarbonatite magma and occurs as minute rims on the alabandite. Its formula is: (K 0.94 Na 0.06 )(Fe 1.91 Mn 0.13 Ca 0.01 Sr 0.01 )(S 2.06 F 0.24 Cl 0.04 ).