Subsolidus Relations in the System Na₂CO₃-CaCO₃-H₂O

Abstract

PRESENT interest in this system stems from the study of the unique ‘natro-carbonatite’ lavas¹ erupted at Oldoinyo Lengai, Tanzania, in 1961 and from the recent discovery of the mineral shortite in a high pressure paragenesis². The present study is concerned only with the P-T range 500 to 1,500 bar, 200 to 500° C, in which only two phases of intermediate composition occur, both of which are anhydrous. One has the composition Na₂CO₃.2CaCO₃ and corresponds in its diffraction pattern to the mineral shortite, originally described from the Green River Formation³ and the subject of a recent structure analysis⁴. The other has the composition Na₂CO₃.CaCO₃, which does not correspond to that of any known mineral. The mineral nyerereite, which was identified but not named by Dawson¹ in the remarkable carbonate lavas of Oldoinyo Lengai, Tanzania, can, however, be regarded as Na₂CO₃.CaCO₃ with substantial substitution of sodium by potassium and minor substitution of carbonate by other anions. Crystals of the synthetic phase Na₂CO₃.CaCO₃ are very small, but by X-ray precession photography we have been able to determine the diffraction symbol as mmmPbca and by applying a least-squares fit to the X-ray powder pattern we have determined the unit cell dimensions a= 10.12, b = 8.76, c= 12.25 Å. The natural mineral nyerereite has the diffraction symbol mmmC.c, unit cell dimensions a=25.25, b=8.81, c= 12,74 Å, and a clearly discernible five-fold superlattice in the ? direction. The unit cell of the synthetic phase, which we shall refer to as 'synthetic nyerereite', thus seems to consist of just two subcells of the mineral structure and, because of the absence of potassium, to have a markedly shorter c repeat.