Abstract: A carbonatite dyke from the Sarnu-Dandali alkaline complex, Rajasthan, India, contains a remarkable suite of rare earth, strontium-rich minerals with spectacular primary textures.

Sr, Mn-rich calcite in the outer 5 mm of the dyke contains exsolved lamellae of carbocernaite, (Ca,Na)(Sr,Ce,Ba)(CO₃)₂, orientated parallel to its twin and cleavage planes. The amount of exsolved carbocernaite increases away from the dyke margin as the Sr content of the calcite increases to a maximum 13 wt.%. Sr levels as high as this in calcite have previously been recorded only in experimental work. The carbocernaite exsolution suggests that Sr-rich calcium carbonate can be a host for major amounts of REE in carbonatite magma.

Separated by a sharp internal boundary, is a complex possibly cotectic intergrowth of carbocernaite and Sr-rich calcite with late Ca-rich strontianite (19 wt.% CaO). Other minerals in the dyke include baryte, pyrrhotite, alabandite, sphalerite and occasional bastnäsite-(La) and thorite. Bands of late britholite-(Ce) traverse the dyke.

The host rock for the dyke is fenitized melanephelinite which is itself traversed by narrow, <1 mm, carbonatite veins beleived to predate the carbonatite dyke. Allanite, britholite-(Ce) and rare monazite-(Ce), developed at the boundary between the carbonatite dyke and the fenite, may have been produced by a reaction between the dyke and the wall rock, or may be related to the later britholite mineralisation.

The textures and mineral compositions indicate primary crystallisation. They are unique amongst rare earth-rich carbonatites which are usually late-stage phenomena with signs of secondary alteration.

Comparison with experimental data available for the calcite-strontianite system suggests conditions of 500°C and 2 kbar for coexisting Sr-rich calcite and Ca-rich strontianite. A smaller scale intergrowth of calcite containing only 2.9 wt.% SrO and coexisting Ca-strontianite may correspond to a further unmixing at 350°C and 2 kbar. Since no experimental data are available for a calcite-carbocernaite-strontianite system, mineral chemistries and the interpreted sequence of crystallisation have been used to construct a hypothetical phase diagram.