Fairchildite K2Ca(CO3)2 in phoscorites from Phalaborwa, South Africa: the first occurrence in alkaline carbonatite complexes

The paper is devoted to monomineralic and multiphase (melt) inclusions in magnetites from the phoscorites of the Loolekop deposit, the Phalaborwa alkaline carbonatite complex, South Africa. Multiphase inclusions vary greatly in phase composition from essentially carbonate to carbonate–silicate–oxide. Their main components are dolomite, calcite, fluorapatite, picroilmenite, phlogopite, magnesite, and brucite. In one multiphase inclusion, a rare carbonate, fairchildite K2Ca(CO3)2, was found in the central part of a large magnetite grain. It coexists with dolomite, picroilmenite, phlogopite, brucite, witherite, and halite. This is the first occurrence of fairchildite in carbonatite complexes and in igneous rocks in general. The composition of this mineral (38.54 wt.% K2O, 23.15 wt.% CaO, 1.47 wt.% FeO, 0.63 wt.% Na2O, n = 5) is close to the ideal K2Ca(CO3)2. The phase relationships within the inclusion show that potassium–calcium carbonate crystallized after phlogopite and picroilmenite, but before dolomite. Temperature estimations from the pairs magnetite–ilmenite and dolomite–calcite indicate that phoscorites began to form at T > 630–750 °C and the phase K2Ca(CO3)2 is a high-temperature modification (fairchildite) which crystallized directly from melt rather than by solid-phase reactions. However, it can be transformed into a low-temperature modification (bütschliite) with decreasing temperature at 547 °C. Apparently, the extremely low Na content and relatively high K content of the initial calciocarbonatite melt favored the formation of fairchildite in the Phalaborwa phoscorites. According to literature data, if Na2O dominates over K2O in the carbonatite melt, nyerereite Na2Ca(CO3)2 is a favorable phase for crystallization and K incorporates into its structure as a minor component.