Mineralogy of Silicate-Natrophosphate Immiscible Inclusion in Elga IIE Iron Meteorite

Rare type of silicate inclusions found in the Elga iron meteorite (group IIE) has a very specific mineral composition and shows silicate (approximate to 90%)-natrophosphate (approximate to 10%) liquid immiscibility due to meniscus-like isolation of Na-Ca-Mg-Fe phosphates. The 3 mm wide immiscible inclusion has been first studied in detail using optical microscopy, scanning electron microscopy, electron microprobe analysis and Raman spectroscopy. The silicate part of the inclusion contains fine-grained quartz-feldspar aggregate and mafic minerals. The relationships of feldspars indicate solid decay of initially homogenous K-Na-feldspar into albite and K-feldspar with decreasing of temperature. Some mafic minerals in the silicate part are exotic in composition: the dominant phase is an obertiite-subgroup oxyamphibole (amphibole supergroup), varying from ferri-obertiite NaNa2Mg3Fe3+Ti[Si8O22]O-2 to hypothetical NaNa2Mg3Fe0.52+Ti1.5[Si8O22]O-2; minor phases are the aenigmatite-subgroup mineral (sapphirine supergroup) with composition close to median value of the Na2Fe52+TiSi6O18O2-Na2Mg5TiSi6O18O2 join, orthopyroxene (enstatite), clinopyroxene of the diopside Ca(Mg,Fe)Si2O6-kosmochlor NaCrSi2O6-Na(Mg,Fe)(0.5)Ti0.5Si2O6 series and chromite. The alteration phases are represented by Fe-dominant chlorite, goethite and hydrated Na2O-rich (2.3-3.3 wt.%) Fe-phosphate close to vivianite. Natrophosphate part consists of aggregate of three orthophosphates (brianite, czochralskiite, mariite) and minor Na-Cr-Ti-clinopyroxene, pentlandite, rarely taenite. Czochralskiite Na4Ca3Mg(PO4)(4) is rich in FeO (2.3-5.1 wt.%) and MnO (0.4-1.5 wt.%). Brianite Na2CaMg(PO4)(2) contains FeO (3.0-4.3 wt.%) and MnO (0.3-0.7 wt.%) and mariite NaFe(PO4) bears MnO (5.5-6.2 wt.%), MgO (5.3-6.2 wt.%) and CaO (0.5-1.5 wt.%). The contact between immiscible parts is decorated by enstatite zone in the silicate part and diopside-kosmochlor clinopyroxene zone in the natrophosphate ones. The mineralogy of the studied immiscible inclusion outlines three potentially new mineral species, which were first identified in meteorites: obertiite-related oxyamphibole NaNa2Mg3Fe0.52+Ti1.5[Si8O22]O-2, Mg-analog of aenigmatite Na2Mg5TiSi6O18O2 and Na-Ti-rich clinopyroxene Na(Mg,Fe)(0.5)Ti0.5Si2O6.