Unusual Mineralogy of Kimberlites: Alkali Carbonates, Sulfates, and Chlorides Among Groundmass Minerals from Unserpentinized Coherent Kimberlite of the Udachnaya-East Pipe, Siberian Craton

The paper reports the first findings of a series of alkali carbonate, chloride, and sulfate minerals among the usual groundmass kimberlite minerals, such as olivine, phlogopite, monticellite, calcite, spinel-group minerals, perovskite, ilmenite, rutile, and apatite. The sample was collected from an unserpentinized coherent kimberlite dyke that crosscuts earlier volcaniclastic kimberlite in the central part of the Udachnaya-East pipe. This rock can be described as primary/original kimberlite that did not interact with external/internal hydrothermal fluids either during its formation or after its crystallization. At least three alkali-rich carbonates have been found, a previously unknown (and perhaps, a new one) Na-, Ca-, K-, and S-rich carbonate with the calculated empirical formula (Na,K)6Ca4(CO3,SO4)7, shortite Na2Ca2(CO3)3, and nyerereite (Na,K)2Ca(CO3)2. Chlorides in this kimberlite are halite NaCl and sylvite KCl, and the sulfate is aphthitalite K3Na(SO4)2. The content of the Na-Ca-K-S-rich carbonate in the rock is ~15 vol %, that of shortite and halite is ≤5 vol % each, and those of sylvite and aphthitalite are ≤1 vol %. All alkali-rich minerals are of late magmatic origin. This follows from that (i) the studied kimberlite does not contain any secondary water-rich minerals of hydrothermal transformation of the rocks, such as serpentine, chlorite or iowaite; and (ii) crystalline inclusions of such usual kimberlite minerals as olivine, phlogopite, monticellite, calcite, spinel, perovskite, and apatite were found within Na-Ca-K-S-rich carbonate and halite. This publication expands the list of minerals of magmatic origin identified in the groundmass of worldwide kimberlites by at least three minerals: Na-Ca-K-S-rich (new?) carbonate, sylvite, and aphthitalite. It is important to note that all alkali carbonates, chlorides, and sulfates are unstable during secondary hydrothermal alterations of kimberlites, and hence, these minerals cannot be found in serpentinized rocks.